Merge tag 'audit-pr-20200729' of git://git.kernel.org/pub/scm/linux/kernel/git/pcmoor...

[linux-2.6-microblaze.git] / fs / xfs / xfs_buf.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (c) 2000-2006 Silicon Graphics, Inc.
 * All Rights Reserved.
 */
#include "xfs.h"
#include <linux/backing-dev.h>

#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_trans_resv.h"
#include "xfs_sb.h"
#include "xfs_mount.h"
#include "xfs_trace.h"
#include "xfs_log.h"
#include "xfs_errortag.h"
#include "xfs_error.h"

static kmem_zone_t *xfs_buf_zone;

#define xb_to_gfp(flags) \
        ((((flags) & XBF_READ_AHEAD) ? __GFP_NORETRY : GFP_NOFS) | __GFP_NOWARN)

/*
 * Locking orders
 *
 * xfs_buf_ioacct_inc:
 * xfs_buf_ioacct_dec:
 *      b_sema (caller holds)
 *        b_lock
 *
 * xfs_buf_stale:
 *      b_sema (caller holds)
 *        b_lock
 *          lru_lock
 *
 * xfs_buf_rele:
 *      b_lock
 *        pag_buf_lock
 *          lru_lock
 *
 * xfs_buftarg_wait_rele
 *      lru_lock
 *        b_lock (trylock due to inversion)
 *
 * xfs_buftarg_isolate
 *      lru_lock
 *        b_lock (trylock due to inversion)
 */

static inline int
xfs_buf_is_vmapped(
        struct xfs_buf  *bp)
{
        /*
         * Return true if the buffer is vmapped.
         *
         * b_addr is null if the buffer is not mapped, but the code is clever
         * enough to know it doesn't have to map a single page, so the check has
         * to be both for b_addr and bp->b_page_count > 1.
         */
        return bp->b_addr && bp->b_page_count > 1;
}

static inline int
xfs_buf_vmap_len(
        struct xfs_buf  *bp)
{
        return (bp->b_page_count * PAGE_SIZE) - bp->b_offset;
}

/*
 * Bump the I/O in flight count on the buftarg if we haven't yet done so for
 * this buffer. The count is incremented once per buffer (per hold cycle)
 * because the corresponding decrement is deferred to buffer release. Buffers
 * can undergo I/O multiple times in a hold-release cycle and per buffer I/O
 * tracking adds unnecessary overhead. This is used for sychronization purposes
 * with unmount (see xfs_wait_buftarg()), so all we really need is a count of
 * in-flight buffers.
 *
 * Buffers that are never released (e.g., superblock, iclog buffers) must set
 * the XBF_NO_IOACCT flag before I/O submission. Otherwise, the buftarg count
 * never reaches zero and unmount hangs indefinitely.
 */
static inline void
xfs_buf_ioacct_inc(
        struct xfs_buf  *bp)
{
        if (bp->b_flags & XBF_NO_IOACCT)
                return;

        ASSERT(bp->b_flags & XBF_ASYNC);
        spin_lock(&bp->b_lock);
        if (!(bp->b_state & XFS_BSTATE_IN_FLIGHT)) {
                bp->b_state |= XFS_BSTATE_IN_FLIGHT;
                percpu_counter_inc(&bp->b_target->bt_io_count);
        }
        spin_unlock(&bp->b_lock);
}

/*
 * Clear the in-flight state on a buffer about to be released to the LRU or
 * freed and unaccount from the buftarg.
 */
static inline void
__xfs_buf_ioacct_dec(
        struct xfs_buf  *bp)
{
        lockdep_assert_held(&bp->b_lock);

        if (bp->b_state & XFS_BSTATE_IN_FLIGHT) {
                bp->b_state &= ~XFS_BSTATE_IN_FLIGHT;
                percpu_counter_dec(&bp->b_target->bt_io_count);
        }
}

static inline void
xfs_buf_ioacct_dec(
        struct xfs_buf  *bp)
{
        spin_lock(&bp->b_lock);
        __xfs_buf_ioacct_dec(bp);
        spin_unlock(&bp->b_lock);
}

/*
 * When we mark a buffer stale, we remove the buffer from the LRU and clear the
 * b_lru_ref count so that the buffer is freed immediately when the buffer
 * reference count falls to zero. If the buffer is already on the LRU, we need
 * to remove the reference that LRU holds on the buffer.
 *
 * This prevents build-up of stale buffers on the LRU.
 */
void
xfs_buf_stale(
        struct xfs_buf  *bp)
{
        ASSERT(xfs_buf_islocked(bp));

        bp->b_flags |= XBF_STALE;

        /*
         * Clear the delwri status so that a delwri queue walker will not
         * flush this buffer to disk now that it is stale. The delwri queue has
         * a reference to the buffer, so this is safe to do.
         */
        bp->b_flags &= ~_XBF_DELWRI_Q;

        /*
         * Once the buffer is marked stale and unlocked, a subsequent lookup
         * could reset b_flags. There is no guarantee that the buffer is
         * unaccounted (released to LRU) before that occurs. Drop in-flight
         * status now to preserve accounting consistency.
         */
        spin_lock(&bp->b_lock);
        __xfs_buf_ioacct_dec(bp);

        atomic_set(&bp->b_lru_ref, 0);
        if (!(bp->b_state & XFS_BSTATE_DISPOSE) &&
            (list_lru_del(&bp->b_target->bt_lru, &bp->b_lru)))
                atomic_dec(&bp->b_hold);

        ASSERT(atomic_read(&bp->b_hold) >= 1);
        spin_unlock(&bp->b_lock);
}

static int
xfs_buf_get_maps(
        struct xfs_buf          *bp,
        int                     map_count)
{
        ASSERT(bp->b_maps == NULL);
        bp->b_map_count = map_count;

        if (map_count == 1) {
                bp->b_maps = &bp->__b_map;
                return 0;
        }

        bp->b_maps = kmem_zalloc(map_count * sizeof(struct xfs_buf_map),
                                KM_NOFS);
        if (!bp->b_maps)
                return -ENOMEM;
        return 0;
}

/*
 *      Frees b_pages if it was allocated.
 */
static void
xfs_buf_free_maps(
        struct xfs_buf  *bp)
{
        if (bp->b_maps != &bp->__b_map) {
                kmem_free(bp->b_maps);
                bp->b_maps = NULL;
        }
}

static int
_xfs_buf_alloc(
        struct xfs_buftarg      *target,
        struct xfs_buf_map      *map,
        int                     nmaps,
        xfs_buf_flags_t         flags,
        struct xfs_buf          **bpp)
{
        struct xfs_buf          *bp;
        int                     error;
        int                     i;

        *bpp = NULL;
        bp = kmem_zone_zalloc(xfs_buf_zone, KM_NOFS);
        if (unlikely(!bp))
                return -ENOMEM;

        /*
         * We don't want certain flags to appear in b_flags unless they are
         * specifically set by later operations on the buffer.
         */
        flags &= ~(XBF_UNMAPPED | XBF_TRYLOCK | XBF_ASYNC | XBF_READ_AHEAD);

        atomic_set(&bp->b_hold, 1);
        atomic_set(&bp->b_lru_ref, 1);
        init_completion(&bp->b_iowait);
        INIT_LIST_HEAD(&bp->b_lru);
        INIT_LIST_HEAD(&bp->b_list);
        INIT_LIST_HEAD(&bp->b_li_list);
        sema_init(&bp->b_sema, 0); /* held, no waiters */
        spin_lock_init(&bp->b_lock);
        bp->b_target = target;
        bp->b_mount = target->bt_mount;
        bp->b_flags = flags;

        /*
         * Set length and io_length to the same value initially.
         * I/O routines should use io_length, which will be the same in
         * most cases but may be reset (e.g. XFS recovery).
         */
        error = xfs_buf_get_maps(bp, nmaps);
        if (error)  {
                kmem_cache_free(xfs_buf_zone, bp);
                return error;
        }

        bp->b_bn = map[0].bm_bn;
        bp->b_length = 0;
        for (i = 0; i < nmaps; i++) {
                bp->b_maps[i].bm_bn = map[i].bm_bn;
                bp->b_maps[i].bm_len = map[i].bm_len;
                bp->b_length += map[i].bm_len;
        }

        atomic_set(&bp->b_pin_count, 0);
        init_waitqueue_head(&bp->b_waiters);

        XFS_STATS_INC(bp->b_mount, xb_create);
        trace_xfs_buf_init(bp, _RET_IP_);

        *bpp = bp;
        return 0;
}

/*
 *      Allocate a page array capable of holding a specified number
 *      of pages, and point the page buf at it.
 */
STATIC int
_xfs_buf_get_pages(
        xfs_buf_t               *bp,
        int                     page_count)
{
        /* Make sure that we have a page list */
        if (bp->b_pages == NULL) {
                bp->b_page_count = page_count;
                if (page_count <= XB_PAGES) {
                        bp->b_pages = bp->b_page_array;
                } else {
                        bp->b_pages = kmem_alloc(sizeof(struct page *) *
                                                 page_count, KM_NOFS);
                        if (bp->b_pages == NULL)
                                return -ENOMEM;
                }
                memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
        }
        return 0;
}

/*
 *      Frees b_pages if it was allocated.
 */
STATIC void
_xfs_buf_free_pages(
        xfs_buf_t       *bp)
{
        if (bp->b_pages != bp->b_page_array) {
                kmem_free(bp->b_pages);
                bp->b_pages = NULL;
        }
}

/*
 *      Releases the specified buffer.
 *
 *      The modification state of any associated pages is left unchanged.
 *      The buffer must not be on any hash - use xfs_buf_rele instead for
 *      hashed and refcounted buffers
 */
static void
xfs_buf_free(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_free(bp, _RET_IP_);

        ASSERT(list_empty(&bp->b_lru));

        if (bp->b_flags & _XBF_PAGES) {
                uint            i;

                if (xfs_buf_is_vmapped(bp))
                        vm_unmap_ram(bp->b_addr - bp->b_offset,
                                        bp->b_page_count);

                for (i = 0; i < bp->b_page_count; i++) {
                        struct page     *page = bp->b_pages[i];

                        __free_page(page);
                }
                if (current->reclaim_state)
                        current->reclaim_state->reclaimed_slab +=
                                                        bp->b_page_count;
        } else if (bp->b_flags & _XBF_KMEM)
                kmem_free(bp->b_addr);
        _xfs_buf_free_pages(bp);
        xfs_buf_free_maps(bp);
        kmem_cache_free(xfs_buf_zone, bp);
}

/*
 * Allocates all the pages for buffer in question and builds it's page list.
 */
STATIC int
xfs_buf_allocate_memory(
        xfs_buf_t               *bp,
        uint                    flags)
{
        size_t                  size;
        size_t                  nbytes, offset;
        gfp_t                   gfp_mask = xb_to_gfp(flags);
        unsigned short          page_count, i;
        xfs_off_t               start, end;
        int                     error;
        xfs_km_flags_t          kmflag_mask = 0;

        /*
         * assure zeroed buffer for non-read cases.
         */
        if (!(flags & XBF_READ)) {
                kmflag_mask |= KM_ZERO;
                gfp_mask |= __GFP_ZERO;
        }

        /*
         * for buffers that are contained within a single page, just allocate
         * the memory from the heap - there's no need for the complexity of
         * page arrays to keep allocation down to order 0.
         */
        size = BBTOB(bp->b_length);
        if (size < PAGE_SIZE) {
                int align_mask = xfs_buftarg_dma_alignment(bp->b_target);
                bp->b_addr = kmem_alloc_io(size, align_mask,
                                           KM_NOFS | kmflag_mask);
                if (!bp->b_addr) {
                        /* low memory - use alloc_page loop instead */
                        goto use_alloc_page;
                }

                if (((unsigned long)(bp->b_addr + size - 1) & PAGE_MASK) !=
                    ((unsigned long)bp->b_addr & PAGE_MASK)) {
                        /* b_addr spans two pages - use alloc_page instead */
                        kmem_free(bp->b_addr);
                        bp->b_addr = NULL;
                        goto use_alloc_page;
                }
                bp->b_offset = offset_in_page(bp->b_addr);
                bp->b_pages = bp->b_page_array;
                bp->b_pages[0] = kmem_to_page(bp->b_addr);
                bp->b_page_count = 1;
                bp->b_flags |= _XBF_KMEM;
                return 0;
        }

use_alloc_page:
        start = BBTOB(bp->b_maps[0].bm_bn) >> PAGE_SHIFT;
        end = (BBTOB(bp->b_maps[0].bm_bn + bp->b_length) + PAGE_SIZE - 1)
                                                                >> PAGE_SHIFT;
        page_count = end - start;
        error = _xfs_buf_get_pages(bp, page_count);
        if (unlikely(error))
                return error;

        offset = bp->b_offset;
        bp->b_flags |= _XBF_PAGES;

        for (i = 0; i < bp->b_page_count; i++) {
                struct page     *page;
                uint            retries = 0;
retry:
                page = alloc_page(gfp_mask);
                if (unlikely(page == NULL)) {
                        if (flags & XBF_READ_AHEAD) {
                                bp->b_page_count = i;
                                error = -ENOMEM;
                                goto out_free_pages;
                        }

                        /*
                         * This could deadlock.
                         *
                         * But until all the XFS lowlevel code is revamped to
                         * handle buffer allocation failures we can't do much.
                         */
                        if (!(++retries % 100))
                                xfs_err(NULL,
                "%s(%u) possible memory allocation deadlock in %s (mode:0x%x)",
                                        current->comm, current->pid,
                                        __func__, gfp_mask);

                        XFS_STATS_INC(bp->b_mount, xb_page_retries);
                        congestion_wait(BLK_RW_ASYNC, HZ/50);
                        goto retry;
                }

                XFS_STATS_INC(bp->b_mount, xb_page_found);

                nbytes = min_t(size_t, size, PAGE_SIZE - offset);
                size -= nbytes;
                bp->b_pages[i] = page;
                offset = 0;
        }
        return 0;

out_free_pages:
        for (i = 0; i < bp->b_page_count; i++)
                __free_page(bp->b_pages[i]);
        bp->b_flags &= ~_XBF_PAGES;
        return error;
}

/*
 *      Map buffer into kernel address-space if necessary.
 */
STATIC int
_xfs_buf_map_pages(
        xfs_buf_t               *bp,
        uint                    flags)
{
        ASSERT(bp->b_flags & _XBF_PAGES);
        if (bp->b_page_count == 1) {
                /* A single page buffer is always mappable */
                bp->b_addr = page_address(bp->b_pages[0]) + bp->b_offset;
        } else if (flags & XBF_UNMAPPED) {
                bp->b_addr = NULL;
        } else {
                int retried = 0;
                unsigned nofs_flag;

                /*
                 * vm_map_ram() will allocate auxiliary structures (e.g.
                 * pagetables) with GFP_KERNEL, yet we are likely to be under
                 * GFP_NOFS context here. Hence we need to tell memory reclaim
                 * that we are in such a context via PF_MEMALLOC_NOFS to prevent
                 * memory reclaim re-entering the filesystem here and
                 * potentially deadlocking.
                 */
                nofs_flag = memalloc_nofs_save();
                do {
                        bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,
                                                -1);
                        if (bp->b_addr)
                                break;
                        vm_unmap_aliases();
                } while (retried++ <= 1);
                memalloc_nofs_restore(nofs_flag);

                if (!bp->b_addr)
                        return -ENOMEM;
                bp->b_addr += bp->b_offset;
        }

        return 0;
}

/*
 *      Finding and Reading Buffers
 */
static int
_xfs_buf_obj_cmp(
        struct rhashtable_compare_arg   *arg,
        const void                      *obj)
{
        const struct xfs_buf_map        *map = arg->key;
        const struct xfs_buf            *bp = obj;

        /*
         * The key hashing in the lookup path depends on the key being the
         * first element of the compare_arg, make sure to assert this.
         */
        BUILD_BUG_ON(offsetof(struct xfs_buf_map, bm_bn) != 0);

        if (bp->b_bn != map->bm_bn)
                return 1;

        if (unlikely(bp->b_length != map->bm_len)) {
                /*
                 * found a block number match. If the range doesn't
                 * match, the only way this is allowed is if the buffer
                 * in the cache is stale and the transaction that made
                 * it stale has not yet committed. i.e. we are
                 * reallocating a busy extent. Skip this buffer and
                 * continue searching for an exact match.
                 */
                ASSERT(bp->b_flags & XBF_STALE);
                return 1;
        }
        return 0;
}

static const struct rhashtable_params xfs_buf_hash_params = {
        .min_size               = 32,   /* empty AGs have minimal footprint */
        .nelem_hint             = 16,
        .key_len                = sizeof(xfs_daddr_t),
        .key_offset             = offsetof(struct xfs_buf, b_bn),
        .head_offset            = offsetof(struct xfs_buf, b_rhash_head),
        .automatic_shrinking    = true,
        .obj_cmpfn              = _xfs_buf_obj_cmp,
};

int
xfs_buf_hash_init(
        struct xfs_perag        *pag)
{
        spin_lock_init(&pag->pag_buf_lock);
        return rhashtable_init(&pag->pag_buf_hash, &xfs_buf_hash_params);
}

void
xfs_buf_hash_destroy(
        struct xfs_perag        *pag)
{
        rhashtable_destroy(&pag->pag_buf_hash);
}

/*
 * Look up a buffer in the buffer cache and return it referenced and locked
 * in @found_bp.
 *
 * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
 * cache.
 *
 * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
 * -EAGAIN if we fail to lock it.
 *
 * Return values are:
 *      -EFSCORRUPTED if have been supplied with an invalid address
 *      -EAGAIN on trylock failure
 *      -ENOENT if we fail to find a match and @new_bp was NULL
 *      0, with @found_bp:
 *              - @new_bp if we inserted it into the cache
 *              - the buffer we found and locked.
 */
static int
xfs_buf_find(
        struct xfs_buftarg      *btp,
        struct xfs_buf_map      *map,
        int                     nmaps,
        xfs_buf_flags_t         flags,
        struct xfs_buf          *new_bp,
        struct xfs_buf          **found_bp)
{
        struct xfs_perag        *pag;
        xfs_buf_t               *bp;
        struct xfs_buf_map      cmap = { .bm_bn = map[0].bm_bn };
        xfs_daddr_t             eofs;
        int                     i;

        *found_bp = NULL;

        for (i = 0; i < nmaps; i++)
                cmap.bm_len += map[i].bm_len;

        /* Check for IOs smaller than the sector size / not sector aligned */
        ASSERT(!(BBTOB(cmap.bm_len) < btp->bt_meta_sectorsize));
        ASSERT(!(BBTOB(cmap.bm_bn) & (xfs_off_t)btp->bt_meta_sectormask));

        /*
         * Corrupted block numbers can get through to here, unfortunately, so we
         * have to check that the buffer falls within the filesystem bounds.
         */
        eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
        if (cmap.bm_bn < 0 || cmap.bm_bn >= eofs) {
                xfs_alert(btp->bt_mount,
                          "%s: daddr 0x%llx out of range, EOFS 0x%llx",
                          __func__, cmap.bm_bn, eofs);
                WARN_ON(1);
                return -EFSCORRUPTED;
        }

        pag = xfs_perag_get(btp->bt_mount,
                            xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));

        spin_lock(&pag->pag_buf_lock);
        bp = rhashtable_lookup_fast(&pag->pag_buf_hash, &cmap,
                                    xfs_buf_hash_params);
        if (bp) {
                atomic_inc(&bp->b_hold);
                goto found;
        }

        /* No match found */
        if (!new_bp) {
                XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
                spin_unlock(&pag->pag_buf_lock);
                xfs_perag_put(pag);
                return -ENOENT;
        }

        /* the buffer keeps the perag reference until it is freed */
        new_bp->b_pag = pag;
        rhashtable_insert_fast(&pag->pag_buf_hash, &new_bp->b_rhash_head,
                               xfs_buf_hash_params);
        spin_unlock(&pag->pag_buf_lock);
        *found_bp = new_bp;
        return 0;

found:
        spin_unlock(&pag->pag_buf_lock);
        xfs_perag_put(pag);

        if (!xfs_buf_trylock(bp)) {
                if (flags & XBF_TRYLOCK) {
                        xfs_buf_rele(bp);
                        XFS_STATS_INC(btp->bt_mount, xb_busy_locked);
                        return -EAGAIN;
                }
                xfs_buf_lock(bp);
                XFS_STATS_INC(btp->bt_mount, xb_get_locked_waited);
        }

        /*
         * if the buffer is stale, clear all the external state associated with
         * it. We need to keep flags such as how we allocated the buffer memory
         * intact here.
         */
        if (bp->b_flags & XBF_STALE) {
                ASSERT((bp->b_flags & _XBF_DELWRI_Q) == 0);
                ASSERT(bp->b_iodone == NULL);
                bp->b_flags &= _XBF_KMEM | _XBF_PAGES;
                bp->b_ops = NULL;
        }

        trace_xfs_buf_find(bp, flags, _RET_IP_);
        XFS_STATS_INC(btp->bt_mount, xb_get_locked);
        *found_bp = bp;
        return 0;
}

struct xfs_buf *
xfs_buf_incore(
        struct xfs_buftarg      *target,
        xfs_daddr_t             blkno,
        size_t                  numblks,
        xfs_buf_flags_t         flags)
{
        struct xfs_buf          *bp;
        int                     error;
        DEFINE_SINGLE_BUF_MAP(map, blkno, numblks);

        error = xfs_buf_find(target, &map, 1, flags, NULL, &bp);
        if (error)
                return NULL;
        return bp;
}

/*
 * Assembles a buffer covering the specified range. The code is optimised for
 * cache hits, as metadata intensive workloads will see 3 orders of magnitude
 * more hits than misses.
 */
int
xfs_buf_get_map(
        struct xfs_buftarg      *target,
        struct xfs_buf_map      *map,
        int                     nmaps,
        xfs_buf_flags_t         flags,
        struct xfs_buf          **bpp)
{
        struct xfs_buf          *bp;
        struct xfs_buf          *new_bp;
        int                     error = 0;

        *bpp = NULL;
        error = xfs_buf_find(target, map, nmaps, flags, NULL, &bp);
        if (!error)
                goto found;
        if (error != -ENOENT)
                return error;

        error = _xfs_buf_alloc(target, map, nmaps, flags, &new_bp);
        if (error)
                return error;

        error = xfs_buf_allocate_memory(new_bp, flags);
        if (error) {
                xfs_buf_free(new_bp);
                return error;
        }

        error = xfs_buf_find(target, map, nmaps, flags, new_bp, &bp);
        if (error) {
                xfs_buf_free(new_bp);
                return error;
        }

        if (bp != new_bp)
                xfs_buf_free(new_bp);

found:
        if (!bp->b_addr) {
                error = _xfs_buf_map_pages(bp, flags);
                if (unlikely(error)) {
                        xfs_warn_ratelimited(target->bt_mount,
                                "%s: failed to map %u pages", __func__,
                                bp->b_page_count);
                        xfs_buf_relse(bp);
                        return error;
                }
        }

        /*
         * Clear b_error if this is a lookup from a caller that doesn't expect
         * valid data to be found in the buffer.
         */
        if (!(flags & XBF_READ))
                xfs_buf_ioerror(bp, 0);

        XFS_STATS_INC(target->bt_mount, xb_get);
        trace_xfs_buf_get(bp, flags, _RET_IP_);
        *bpp = bp;
        return 0;
}

STATIC int
_xfs_buf_read(
        xfs_buf_t               *bp,
        xfs_buf_flags_t         flags)
{
        ASSERT(!(flags & XBF_WRITE));
        ASSERT(bp->b_maps[0].bm_bn != XFS_BUF_DADDR_NULL);

        bp->b_flags &= ~(XBF_WRITE | XBF_ASYNC | XBF_READ_AHEAD);
        bp->b_flags |= flags & (XBF_READ | XBF_ASYNC | XBF_READ_AHEAD);

        return xfs_buf_submit(bp);
}

/*
 * Reverify a buffer found in cache without an attached ->b_ops.
 *
 * If the caller passed an ops structure and the buffer doesn't have ops
 * assigned, set the ops and use it to verify the contents. If verification
 * fails, clear XBF_DONE. We assume the buffer has no recorded errors and is
 * already in XBF_DONE state on entry.
 *
 * Under normal operations, every in-core buffer is verified on read I/O
 * completion. There are two scenarios that can lead to in-core buffers without
 * an assigned ->b_ops. The first is during log recovery of buffers on a V4
 * filesystem, though these buffers are purged at the end of recovery. The
 * other is online repair, which intentionally reads with a NULL buffer ops to
 * run several verifiers across an in-core buffer in order to establish buffer
 * type.  If repair can't establish that, the buffer will be left in memory
 * with NULL buffer ops.
 */
int
xfs_buf_reverify(
        struct xfs_buf          *bp,
        const struct xfs_buf_ops *ops)
{
        ASSERT(bp->b_flags & XBF_DONE);
        ASSERT(bp->b_error == 0);

        if (!ops || bp->b_ops)
                return 0;

        bp->b_ops = ops;
        bp->b_ops->verify_read(bp);
        if (bp->b_error)
                bp->b_flags &= ~XBF_DONE;
        return bp->b_error;
}

int
xfs_buf_read_map(
        struct xfs_buftarg      *target,
        struct xfs_buf_map      *map,
        int                     nmaps,
        xfs_buf_flags_t         flags,
        struct xfs_buf          **bpp,
        const struct xfs_buf_ops *ops,
        xfs_failaddr_t          fa)
{
        struct xfs_buf          *bp;
        int                     error;

        flags |= XBF_READ;
        *bpp = NULL;

        error = xfs_buf_get_map(target, map, nmaps, flags, &bp);
        if (error)
                return error;

        trace_xfs_buf_read(bp, flags, _RET_IP_);

        if (!(bp->b_flags & XBF_DONE)) {
                /* Initiate the buffer read and wait. */
                XFS_STATS_INC(target->bt_mount, xb_get_read);
                bp->b_ops = ops;
                error = _xfs_buf_read(bp, flags);

                /* Readahead iodone already dropped the buffer, so exit. */
                if (flags & XBF_ASYNC)
                        return 0;
        } else {
                /* Buffer already read; all we need to do is check it. */
                error = xfs_buf_reverify(bp, ops);

                /* Readahead already finished; drop the buffer and exit. */
                if (flags & XBF_ASYNC) {
                        xfs_buf_relse(bp);
                        return 0;
                }

                /* We do not want read in the flags */
                bp->b_flags &= ~XBF_READ;
                ASSERT(bp->b_ops != NULL || ops == NULL);
        }

        /*
         * If we've had a read error, then the contents of the buffer are
         * invalid and should not be used. To ensure that a followup read tries
         * to pull the buffer from disk again, we clear the XBF_DONE flag and
         * mark the buffer stale. This ensures that anyone who has a current
         * reference to the buffer will interpret it's contents correctly and
         * future cache lookups will also treat it as an empty, uninitialised
         * buffer.
         */
        if (error) {
                if (!XFS_FORCED_SHUTDOWN(target->bt_mount))
                        xfs_buf_ioerror_alert(bp, fa);

                bp->b_flags &= ~XBF_DONE;
                xfs_buf_stale(bp);
                xfs_buf_relse(bp);

                /* bad CRC means corrupted metadata */
                if (error == -EFSBADCRC)
                        error = -EFSCORRUPTED;
                return error;
        }

        *bpp = bp;
        return 0;
}

/*
 *      If we are not low on memory then do the readahead in a deadlock
 *      safe manner.
 */
void
xfs_buf_readahead_map(
        struct xfs_buftarg      *target,
        struct xfs_buf_map      *map,
        int                     nmaps,
        const struct xfs_buf_ops *ops)
{
        struct xfs_buf          *bp;

        if (bdi_read_congested(target->bt_bdev->bd_bdi))
                return;

        xfs_buf_read_map(target, map, nmaps,
                     XBF_TRYLOCK | XBF_ASYNC | XBF_READ_AHEAD, &bp, ops,
                     __this_address);
}

/*
 * Read an uncached buffer from disk. Allocates and returns a locked
 * buffer containing the disk contents or nothing.
 */
int
xfs_buf_read_uncached(
        struct xfs_buftarg      *target,
        xfs_daddr_t             daddr,
        size_t                  numblks,
        int                     flags,
        struct xfs_buf          **bpp,
        const struct xfs_buf_ops *ops)
{
        struct xfs_buf          *bp;
        int                     error;

        *bpp = NULL;

        error = xfs_buf_get_uncached(target, numblks, flags, &bp);
        if (error)
                return error;

        /* set up the buffer for a read IO */
        ASSERT(bp->b_map_count == 1);
        bp->b_bn = XFS_BUF_DADDR_NULL;  /* always null for uncached buffers */
        bp->b_maps[0].bm_bn = daddr;
        bp->b_flags |= XBF_READ;
        bp->b_ops = ops;

        xfs_buf_submit(bp);
        if (bp->b_error) {
                error = bp->b_error;
                xfs_buf_relse(bp);
                return error;
        }

        *bpp = bp;
        return 0;
}

int
xfs_buf_get_uncached(
        struct xfs_buftarg      *target,
        size_t                  numblks,
        int                     flags,
        struct xfs_buf          **bpp)
{
        unsigned long           page_count;
        int                     error, i;
        struct xfs_buf          *bp;
        DEFINE_SINGLE_BUF_MAP(map, XFS_BUF_DADDR_NULL, numblks);

        *bpp = NULL;

        /* flags might contain irrelevant bits, pass only what we care about */
        error = _xfs_buf_alloc(target, &map, 1, flags & XBF_NO_IOACCT, &bp);
        if (error)
                goto fail;

        page_count = PAGE_ALIGN(numblks << BBSHIFT) >> PAGE_SHIFT;
        error = _xfs_buf_get_pages(bp, page_count);
        if (error)
                goto fail_free_buf;

        for (i = 0; i < page_count; i++) {
                bp->b_pages[i] = alloc_page(xb_to_gfp(flags));
                if (!bp->b_pages[i]) {
                        error = -ENOMEM;
                        goto fail_free_mem;
                }
        }
        bp->b_flags |= _XBF_PAGES;

        error = _xfs_buf_map_pages(bp, 0);
        if (unlikely(error)) {
                xfs_warn(target->bt_mount,
                        "%s: failed to map pages", __func__);
                goto fail_free_mem;
        }

        trace_xfs_buf_get_uncached(bp, _RET_IP_);
        *bpp = bp;
        return 0;

 fail_free_mem:
        while (--i >= 0)
                __free_page(bp->b_pages[i]);
        _xfs_buf_free_pages(bp);
 fail_free_buf:
        xfs_buf_free_maps(bp);
        kmem_cache_free(xfs_buf_zone, bp);
 fail:
        return error;
}

/*
 *      Increment reference count on buffer, to hold the buffer concurrently
 *      with another thread which may release (free) the buffer asynchronously.
 *      Must hold the buffer already to call this function.
 */
void
xfs_buf_hold(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_hold(bp, _RET_IP_);
        atomic_inc(&bp->b_hold);
}

/*
 * Release a hold on the specified buffer. If the hold count is 1, the buffer is
 * placed on LRU or freed (depending on b_lru_ref).
 */
void
xfs_buf_rele(
        xfs_buf_t               *bp)
{
        struct xfs_perag        *pag = bp->b_pag;
        bool                    release;
        bool                    freebuf = false;

        trace_xfs_buf_rele(bp, _RET_IP_);

        if (!pag) {
                ASSERT(list_empty(&bp->b_lru));
                if (atomic_dec_and_test(&bp->b_hold)) {
                        xfs_buf_ioacct_dec(bp);
                        xfs_buf_free(bp);
                }
                return;
        }

        ASSERT(atomic_read(&bp->b_hold) > 0);

        /*
         * We grab the b_lock here first to serialise racing xfs_buf_rele()
         * calls. The pag_buf_lock being taken on the last reference only
         * serialises against racing lookups in xfs_buf_find(). IOWs, the second
         * to last reference we drop here is not serialised against the last
         * reference until we take bp->b_lock. Hence if we don't grab b_lock
         * first, the last "release" reference can win the race to the lock and
         * free the buffer before the second-to-last reference is processed,
         * leading to a use-after-free scenario.
         */
        spin_lock(&bp->b_lock);
        release = atomic_dec_and_lock(&bp->b_hold, &pag->pag_buf_lock);
        if (!release) {
                /*
                 * Drop the in-flight state if the buffer is already on the LRU
                 * and it holds the only reference. This is racy because we
                 * haven't acquired the pag lock, but the use of _XBF_IN_FLIGHT
                 * ensures the decrement occurs only once per-buf.
                 */
                if ((atomic_read(&bp->b_hold) == 1) && !list_empty(&bp->b_lru))
                        __xfs_buf_ioacct_dec(bp);
                goto out_unlock;
        }

        /* the last reference has been dropped ... */
        __xfs_buf_ioacct_dec(bp);
        if (!(bp->b_flags & XBF_STALE) && atomic_read(&bp->b_lru_ref)) {
                /*
                 * If the buffer is added to the LRU take a new reference to the
                 * buffer for the LRU and clear the (now stale) dispose list
                 * state flag
                 */
                if (list_lru_add(&bp->b_target->bt_lru, &bp->b_lru)) {
                        bp->b_state &= ~XFS_BSTATE_DISPOSE;
                        atomic_inc(&bp->b_hold);
                }
                spin_unlock(&pag->pag_buf_lock);
        } else {
                /*
                 * most of the time buffers will already be removed from the
                 * LRU, so optimise that case by checking for the
                 * XFS_BSTATE_DISPOSE flag indicating the last list the buffer
                 * was on was the disposal list
                 */
                if (!(bp->b_state & XFS_BSTATE_DISPOSE)) {
                        list_lru_del(&bp->b_target->bt_lru, &bp->b_lru);
                } else {
                        ASSERT(list_empty(&bp->b_lru));
                }

                ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));
                rhashtable_remove_fast(&pag->pag_buf_hash, &bp->b_rhash_head,
                                       xfs_buf_hash_params);
                spin_unlock(&pag->pag_buf_lock);
                xfs_perag_put(pag);
                freebuf = true;
        }

out_unlock:
        spin_unlock(&bp->b_lock);

        if (freebuf)
                xfs_buf_free(bp);
}


/*
 *      Lock a buffer object, if it is not already locked.
 *
 *      If we come across a stale, pinned, locked buffer, we know that we are
 *      being asked to lock a buffer that has been reallocated. Because it is
 *      pinned, we know that the log has not been pushed to disk and hence it
 *      will still be locked.  Rather than continuing to have trylock attempts
 *      fail until someone else pushes the log, push it ourselves before
 *      returning.  This means that the xfsaild will not get stuck trying
 *      to push on stale inode buffers.
 */
int
xfs_buf_trylock(
        struct xfs_buf          *bp)
{
        int                     locked;

        locked = down_trylock(&bp->b_sema) == 0;
        if (locked)
                trace_xfs_buf_trylock(bp, _RET_IP_);
        else
                trace_xfs_buf_trylock_fail(bp, _RET_IP_);
        return locked;
}

/*
 *      Lock a buffer object.
 *
 *      If we come across a stale, pinned, locked buffer, we know that we
 *      are being asked to lock a buffer that has been reallocated. Because
 *      it is pinned, we know that the log has not been pushed to disk and
 *      hence it will still be locked. Rather than sleeping until someone
 *      else pushes the log, push it ourselves before trying to get the lock.
 */
void
xfs_buf_lock(
        struct xfs_buf          *bp)
{
        trace_xfs_buf_lock(bp, _RET_IP_);

        if (atomic_read(&bp->b_pin_count) && (bp->b_flags & XBF_STALE))
                xfs_log_force(bp->b_mount, 0);
        down(&bp->b_sema);

        trace_xfs_buf_lock_done(bp, _RET_IP_);
}

void
xfs_buf_unlock(
        struct xfs_buf          *bp)
{
        ASSERT(xfs_buf_islocked(bp));

        up(&bp->b_sema);
        trace_xfs_buf_unlock(bp, _RET_IP_);
}

STATIC void
xfs_buf_wait_unpin(
        xfs_buf_t               *bp)
{
        DECLARE_WAITQUEUE       (wait, current);

        if (atomic_read(&bp->b_pin_count) == 0)
                return;

        add_wait_queue(&bp->b_waiters, &wait);
        for (;;) {
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (atomic_read(&bp->b_pin_count) == 0)
                        break;
                io_schedule();
        }
        remove_wait_queue(&bp->b_waiters, &wait);
        set_current_state(TASK_RUNNING);
}

/*
 *      Buffer Utility Routines
 */

void
xfs_buf_ioend(
        struct xfs_buf  *bp)
{
        bool            read = bp->b_flags & XBF_READ;

        trace_xfs_buf_iodone(bp, _RET_IP_);

        bp->b_flags &= ~(XBF_READ | XBF_WRITE | XBF_READ_AHEAD);

        /*
         * Pull in IO completion errors now. We are guaranteed to be running
         * single threaded, so we don't need the lock to read b_io_error.
         */
        if (!bp->b_error && bp->b_io_error)
                xfs_buf_ioerror(bp, bp->b_io_error);

        /* Only validate buffers that were read without errors */
        if (read && !bp->b_error && bp->b_ops) {
                ASSERT(!bp->b_iodone);
                bp->b_ops->verify_read(bp);
        }

        if (!bp->b_error) {
                bp->b_flags &= ~XBF_WRITE_FAIL;
                bp->b_flags |= XBF_DONE;
        }

        if (bp->b_iodone)
                (*(bp->b_iodone))(bp);
        else if (bp->b_flags & XBF_ASYNC)
                xfs_buf_relse(bp);
        else
                complete(&bp->b_iowait);
}

static void
xfs_buf_ioend_work(
        struct work_struct      *work)
{
        struct xfs_buf          *bp =
                container_of(work, xfs_buf_t, b_ioend_work);

        xfs_buf_ioend(bp);
}

static void
xfs_buf_ioend_async(
        struct xfs_buf  *bp)
{
        INIT_WORK(&bp->b_ioend_work, xfs_buf_ioend_work);
        queue_work(bp->b_mount->m_buf_workqueue, &bp->b_ioend_work);
}

void
__xfs_buf_ioerror(
        xfs_buf_t               *bp,
        int                     error,
        xfs_failaddr_t          failaddr)
{
        ASSERT(error <= 0 && error >= -1000);
        bp->b_error = error;
        trace_xfs_buf_ioerror(bp, error, failaddr);
}

void
xfs_buf_ioerror_alert(
        struct xfs_buf          *bp,
        xfs_failaddr_t          func)
{
        xfs_buf_alert_ratelimited(bp, "XFS: metadata IO error",
                "metadata I/O error in \"%pS\" at daddr 0x%llx len %d error %d",
                                  func, (uint64_t)XFS_BUF_ADDR(bp),
                                  bp->b_length, -bp->b_error);
}

/*
 * To simulate an I/O failure, the buffer must be locked and held with at least
 * three references. The LRU reference is dropped by the stale call. The buf
 * item reference is dropped via ioend processing. The third reference is owned
 * by the caller and is dropped on I/O completion if the buffer is XBF_ASYNC.
 */
void
xfs_buf_ioend_fail(
        struct xfs_buf  *bp)
{
        bp->b_flags &= ~XBF_DONE;
        xfs_buf_stale(bp);
        xfs_buf_ioerror(bp, -EIO);
        xfs_buf_ioend(bp);
}

int
xfs_bwrite(
        struct xfs_buf          *bp)
{
        int                     error;

        ASSERT(xfs_buf_islocked(bp));

        bp->b_flags |= XBF_WRITE;
        bp->b_flags &= ~(XBF_ASYNC | XBF_READ | _XBF_DELWRI_Q |
                         XBF_DONE);

        error = xfs_buf_submit(bp);
        if (error)
                xfs_force_shutdown(bp->b_mount, SHUTDOWN_META_IO_ERROR);
        return error;
}

static void
xfs_buf_bio_end_io(
        struct bio              *bio)
{
        struct xfs_buf          *bp = (struct xfs_buf *)bio->bi_private;

        if (!bio->bi_status &&
            (bp->b_flags & XBF_WRITE) && (bp->b_flags & XBF_ASYNC) &&
            XFS_TEST_ERROR(false, bp->b_mount, XFS_ERRTAG_BUF_IOERROR))
                bio->bi_status = BLK_STS_IOERR;

        /*
         * don't overwrite existing errors - otherwise we can lose errors on
         * buffers that require multiple bios to complete.
         */
        if (bio->bi_status) {
                int error = blk_status_to_errno(bio->bi_status);

                cmpxchg(&bp->b_io_error, 0, error);
        }

        if (!bp->b_error && xfs_buf_is_vmapped(bp) && (bp->b_flags & XBF_READ))
                invalidate_kernel_vmap_range(bp->b_addr, xfs_buf_vmap_len(bp));

        if (atomic_dec_and_test(&bp->b_io_remaining) == 1)
                xfs_buf_ioend_async(bp);
        bio_put(bio);
}

static void
xfs_buf_ioapply_map(
        struct xfs_buf  *bp,
        int             map,
        int             *buf_offset,
        int             *count,
        int             op)
{
        int             page_index;
        int             total_nr_pages = bp->b_page_count;
        int             nr_pages;
        struct bio      *bio;
        sector_t        sector =  bp->b_maps[map].bm_bn;
        int             size;
        int             offset;

        /* skip the pages in the buffer before the start offset */
        page_index = 0;
        offset = *buf_offset;
        while (offset >= PAGE_SIZE) {
                page_index++;
                offset -= PAGE_SIZE;
        }

        /*
         * Limit the IO size to the length of the current vector, and update the
         * remaining IO count for the next time around.
         */
        size = min_t(int, BBTOB(bp->b_maps[map].bm_len), *count);
        *count -= size;
        *buf_offset += size;

next_chunk:
        atomic_inc(&bp->b_io_remaining);
        nr_pages = min(total_nr_pages, BIO_MAX_PAGES);

        bio = bio_alloc(GFP_NOIO, nr_pages);
        bio_set_dev(bio, bp->b_target->bt_bdev);
        bio->bi_iter.bi_sector = sector;
        bio->bi_end_io = xfs_buf_bio_end_io;
        bio->bi_private = bp;
        bio->bi_opf = op;

        for (; size && nr_pages; nr_pages--, page_index++) {
                int     rbytes, nbytes = PAGE_SIZE - offset;

                if (nbytes > size)
                        nbytes = size;

                rbytes = bio_add_page(bio, bp->b_pages[page_index], nbytes,
                                      offset);
                if (rbytes < nbytes)
                        break;

                offset = 0;
                sector += BTOBB(nbytes);
                size -= nbytes;
                total_nr_pages--;
        }

        if (likely(bio->bi_iter.bi_size)) {
                if (xfs_buf_is_vmapped(bp)) {
                        flush_kernel_vmap_range(bp->b_addr,
                                                xfs_buf_vmap_len(bp));
                }
                submit_bio(bio);
                if (size)
                        goto next_chunk;
        } else {
                /*
                 * This is guaranteed not to be the last io reference count
                 * because the caller (xfs_buf_submit) holds a count itself.
                 */
                atomic_dec(&bp->b_io_remaining);
                xfs_buf_ioerror(bp, -EIO);
                bio_put(bio);
        }

}

STATIC void
_xfs_buf_ioapply(
        struct xfs_buf  *bp)
{
        struct blk_plug plug;
        int             op;
        int             offset;
        int             size;
        int             i;

        /*
         * Make sure we capture only current IO errors rather than stale errors
         * left over from previous use of the buffer (e.g. failed readahead).
         */
        bp->b_error = 0;

        if (bp->b_flags & XBF_WRITE) {
                op = REQ_OP_WRITE;

                /*
                 * Run the write verifier callback function if it exists. If
                 * this function fails it will mark the buffer with an error and
                 * the IO should not be dispatched.
                 */
                if (bp->b_ops) {
                        bp->b_ops->verify_write(bp);
                        if (bp->b_error) {
                                xfs_force_shutdown(bp->b_mount,
                                                   SHUTDOWN_CORRUPT_INCORE);
                                return;
                        }
                } else if (bp->b_bn != XFS_BUF_DADDR_NULL) {
                        struct xfs_mount *mp = bp->b_mount;

                        /*
                         * non-crc filesystems don't attach verifiers during
                         * log recovery, so don't warn for such filesystems.
                         */
                        if (xfs_sb_version_hascrc(&mp->m_sb)) {
                                xfs_warn(mp,
                                        "%s: no buf ops on daddr 0x%llx len %d",
                                        __func__, bp->b_bn, bp->b_length);
                                xfs_hex_dump(bp->b_addr,
                                                XFS_CORRUPTION_DUMP_LEN);
                                dump_stack();
                        }
                }
        } else {
                op = REQ_OP_READ;
                if (bp->b_flags & XBF_READ_AHEAD)
                        op |= REQ_RAHEAD;
        }

        /* we only use the buffer cache for meta-data */
        op |= REQ_META;

        /*
         * Walk all the vectors issuing IO on them. Set up the initial offset
         * into the buffer and the desired IO size before we start -
         * _xfs_buf_ioapply_vec() will modify them appropriately for each
         * subsequent call.
         */
        offset = bp->b_offset;
        size = BBTOB(bp->b_length);
        blk_start_plug(&plug);
        for (i = 0; i < bp->b_map_count; i++) {
                xfs_buf_ioapply_map(bp, i, &offset, &size, op);
                if (bp->b_error)
                        break;
                if (size <= 0)
                        break;  /* all done */
        }
        blk_finish_plug(&plug);
}

/*
 * Wait for I/O completion of a sync buffer and return the I/O error code.
 */
static int
xfs_buf_iowait(
        struct xfs_buf  *bp)
{
        ASSERT(!(bp->b_flags & XBF_ASYNC));

        trace_xfs_buf_iowait(bp, _RET_IP_);
        wait_for_completion(&bp->b_iowait);
        trace_xfs_buf_iowait_done(bp, _RET_IP_);

        return bp->b_error;
}

/*
 * Buffer I/O submission path, read or write. Asynchronous submission transfers
 * the buffer lock ownership and the current reference to the IO. It is not
 * safe to reference the buffer after a call to this function unless the caller
 * holds an additional reference itself.
 */
int
__xfs_buf_submit(
        struct xfs_buf  *bp,
        bool            wait)
{
        int             error = 0;

        trace_xfs_buf_submit(bp, _RET_IP_);

        ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));

        /* on shutdown we stale and complete the buffer immediately */
        if (XFS_FORCED_SHUTDOWN(bp->b_mount)) {
                xfs_buf_ioend_fail(bp);
                return -EIO;
        }

        /*
         * Grab a reference so the buffer does not go away underneath us. For
         * async buffers, I/O completion drops the callers reference, which
         * could occur before submission returns.
         */
        xfs_buf_hold(bp);

        if (bp->b_flags & XBF_WRITE)
                xfs_buf_wait_unpin(bp);

        /* clear the internal error state to avoid spurious errors */
        bp->b_io_error = 0;

        /*
         * Set the count to 1 initially, this will stop an I/O completion
         * callout which happens before we have started all the I/O from calling
         * xfs_buf_ioend too early.
         */
        atomic_set(&bp->b_io_remaining, 1);
        if (bp->b_flags & XBF_ASYNC)
                xfs_buf_ioacct_inc(bp);
        _xfs_buf_ioapply(bp);

        /*
         * If _xfs_buf_ioapply failed, we can get back here with only the IO
         * reference we took above. If we drop it to zero, run completion so
         * that we don't return to the caller with completion still pending.
         */
        if (atomic_dec_and_test(&bp->b_io_remaining) == 1) {
                if (bp->b_error || !(bp->b_flags & XBF_ASYNC))
                        xfs_buf_ioend(bp);
                else
                        xfs_buf_ioend_async(bp);
        }

        if (wait)
                error = xfs_buf_iowait(bp);

        /*
         * Release the hold that keeps the buffer referenced for the entire
         * I/O. Note that if the buffer is async, it is not safe to reference
         * after this release.
         */
        xfs_buf_rele(bp);
        return error;
}

void *
xfs_buf_offset(
        struct xfs_buf          *bp,
        size_t                  offset)
{
        struct page             *page;

        if (bp->b_addr)
                return bp->b_addr + offset;

        offset += bp->b_offset;
        page = bp->b_pages[offset >> PAGE_SHIFT];
        return page_address(page) + (offset & (PAGE_SIZE-1));
}

void
xfs_buf_zero(
        struct xfs_buf          *bp,
        size_t                  boff,
        size_t                  bsize)
{
        size_t                  bend;

        bend = boff + bsize;
        while (boff < bend) {
                struct page     *page;
                int             page_index, page_offset, csize;

                page_index = (boff + bp->b_offset) >> PAGE_SHIFT;
                page_offset = (boff + bp->b_offset) & ~PAGE_MASK;
                page = bp->b_pages[page_index];
                csize = min_t(size_t, PAGE_SIZE - page_offset,
                                      BBTOB(bp->b_length) - boff);

                ASSERT((csize + page_offset) <= PAGE_SIZE);

                memset(page_address(page) + page_offset, 0, csize);

                boff += csize;
        }
}

/*
 * Log a message about and stale a buffer that a caller has decided is corrupt.
 *
 * This function should be called for the kinds of metadata corruption that
 * cannot be detect from a verifier, such as incorrect inter-block relationship
 * data.  Do /not/ call this function from a verifier function.
 *
 * The buffer must be XBF_DONE prior to the call.  Afterwards, the buffer will
 * be marked stale, but b_error will not be set.  The caller is responsible for
 * releasing the buffer or fixing it.
 */
void
__xfs_buf_mark_corrupt(
        struct xfs_buf          *bp,
        xfs_failaddr_t          fa)
{
        ASSERT(bp->b_flags & XBF_DONE);

        xfs_buf_corruption_error(bp, fa);
        xfs_buf_stale(bp);
}

/*
 *      Handling of buffer targets (buftargs).
 */

/*
 * Wait for any bufs with callbacks that have been submitted but have not yet
 * returned. These buffers will have an elevated hold count, so wait on those
 * while freeing all the buffers only held by the LRU.
 */
static enum lru_status
xfs_buftarg_wait_rele(
        struct list_head        *item,
        struct list_lru_one     *lru,
        spinlock_t              *lru_lock,
        void                    *arg)

{
        struct xfs_buf          *bp = container_of(item, struct xfs_buf, b_lru);
        struct list_head        *dispose = arg;

        if (atomic_read(&bp->b_hold) > 1) {
                /* need to wait, so skip it this pass */
                trace_xfs_buf_wait_buftarg(bp, _RET_IP_);
                return LRU_SKIP;
        }
        if (!spin_trylock(&bp->b_lock))
                return LRU_SKIP;

        /*
         * clear the LRU reference count so the buffer doesn't get
         * ignored in xfs_buf_rele().
         */
        atomic_set(&bp->b_lru_ref, 0);
        bp->b_state |= XFS_BSTATE_DISPOSE;
        list_lru_isolate_move(lru, item, dispose);
        spin_unlock(&bp->b_lock);
        return LRU_REMOVED;
}

void
xfs_wait_buftarg(
        struct xfs_buftarg      *btp)
{
        LIST_HEAD(dispose);
        int                     loop = 0;
        bool                    write_fail = false;

        /*
         * First wait on the buftarg I/O count for all in-flight buffers to be
         * released. This is critical as new buffers do not make the LRU until
         * they are released.
         *
         * Next, flush the buffer workqueue to ensure all completion processing
         * has finished. Just waiting on buffer locks is not sufficient for
         * async IO as the reference count held over IO is not released until
         * after the buffer lock is dropped. Hence we need to ensure here that
         * all reference counts have been dropped before we start walking the
         * LRU list.
         */
        while (percpu_counter_sum(&btp->bt_io_count))
                delay(100);
        flush_workqueue(btp->bt_mount->m_buf_workqueue);

        /* loop until there is nothing left on the lru list. */
        while (list_lru_count(&btp->bt_lru)) {
                list_lru_walk(&btp->bt_lru, xfs_buftarg_wait_rele,
                              &dispose, LONG_MAX);

                while (!list_empty(&dispose)) {
                        struct xfs_buf *bp;
                        bp = list_first_entry(&dispose, struct xfs_buf, b_lru);
                        list_del_init(&bp->b_lru);
                        if (bp->b_flags & XBF_WRITE_FAIL) {
                                write_fail = true;
                                xfs_buf_alert_ratelimited(bp,
                                        "XFS: Corruption Alert",
"Corruption Alert: Buffer at daddr 0x%llx had permanent write failures!",
                                        (long long)bp->b_bn);
                        }
                        xfs_buf_rele(bp);
                }
                if (loop++ != 0)
                        delay(100);
        }

        /*
         * If one or more failed buffers were freed, that means dirty metadata
         * was thrown away. This should only ever happen after I/O completion
         * handling has elevated I/O error(s) to permanent failures and shuts
         * down the fs.
         */
        if (write_fail) {
                ASSERT(XFS_FORCED_SHUTDOWN(btp->bt_mount));
                xfs_alert(btp->bt_mount,
              "Please run xfs_repair to determine the extent of the problem.");
        }
}

static enum lru_status
xfs_buftarg_isolate(
        struct list_head        *item,
        struct list_lru_one     *lru,
        spinlock_t              *lru_lock,
        void                    *arg)
{
        struct xfs_buf          *bp = container_of(item, struct xfs_buf, b_lru);
        struct list_head        *dispose = arg;

        /*
         * we are inverting the lru lock/bp->b_lock here, so use a trylock.
         * If we fail to get the lock, just skip it.
         */
        if (!spin_trylock(&bp->b_lock))
                return LRU_SKIP;
        /*
         * Decrement the b_lru_ref count unless the value is already
         * zero. If the value is already zero, we need to reclaim the
         * buffer, otherwise it gets another trip through the LRU.
         */
        if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
                spin_unlock(&bp->b_lock);
                return LRU_ROTATE;
        }

        bp->b_state |= XFS_BSTATE_DISPOSE;
        list_lru_isolate_move(lru, item, dispose);
        spin_unlock(&bp->b_lock);
        return LRU_REMOVED;
}

static unsigned long
xfs_buftarg_shrink_scan(
        struct shrinker         *shrink,
        struct shrink_control   *sc)
{
        struct xfs_buftarg      *btp = container_of(shrink,
                                        struct xfs_buftarg, bt_shrinker);
        LIST_HEAD(dispose);
        unsigned long           freed;

        freed = list_lru_shrink_walk(&btp->bt_lru, sc,
                                     xfs_buftarg_isolate, &dispose);

        while (!list_empty(&dispose)) {
                struct xfs_buf *bp;
                bp = list_first_entry(&dispose, struct xfs_buf, b_lru);
                list_del_init(&bp->b_lru);
                xfs_buf_rele(bp);
        }

        return freed;
}

static unsigned long
xfs_buftarg_shrink_count(
        struct shrinker         *shrink,
        struct shrink_control   *sc)
{
        struct xfs_buftarg      *btp = container_of(shrink,
                                        struct xfs_buftarg, bt_shrinker);
        return list_lru_shrink_count(&btp->bt_lru, sc);
}

void
xfs_free_buftarg(
        struct xfs_buftarg      *btp)
{
        unregister_shrinker(&btp->bt_shrinker);
        ASSERT(percpu_counter_sum(&btp->bt_io_count) == 0);
        percpu_counter_destroy(&btp->bt_io_count);
        list_lru_destroy(&btp->bt_lru);

        xfs_blkdev_issue_flush(btp);

        kmem_free(btp);
}

int
xfs_setsize_buftarg(
        xfs_buftarg_t           *btp,
        unsigned int            sectorsize)
{
        /* Set up metadata sector size info */
        btp->bt_meta_sectorsize = sectorsize;
        btp->bt_meta_sectormask = sectorsize - 1;

        if (set_blocksize(btp->bt_bdev, sectorsize)) {
                xfs_warn(btp->bt_mount,
                        "Cannot set_blocksize to %u on device %pg",
                        sectorsize, btp->bt_bdev);
                return -EINVAL;
        }

        /* Set up device logical sector size mask */
        btp->bt_logical_sectorsize = bdev_logical_block_size(btp->bt_bdev);
        btp->bt_logical_sectormask = bdev_logical_block_size(btp->bt_bdev) - 1;

        return 0;
}

/*
 * When allocating the initial buffer target we have not yet
 * read in the superblock, so don't know what sized sectors
 * are being used at this early stage.  Play safe.
 */
STATIC int
xfs_setsize_buftarg_early(
        xfs_buftarg_t           *btp,
        struct block_device     *bdev)
{
        return xfs_setsize_buftarg(btp, bdev_logical_block_size(bdev));
}

xfs_buftarg_t *
xfs_alloc_buftarg(
        struct xfs_mount        *mp,
        struct block_device     *bdev,
        struct dax_device       *dax_dev)
{
        xfs_buftarg_t           *btp;

        btp = kmem_zalloc(sizeof(*btp), KM_NOFS);

        btp->bt_mount = mp;
        btp->bt_dev =  bdev->bd_dev;
        btp->bt_bdev = bdev;
        btp->bt_daxdev = dax_dev;

        /*
         * Buffer IO error rate limiting. Limit it to no more than 10 messages
         * per 30 seconds so as to not spam logs too much on repeated errors.
         */
        ratelimit_state_init(&btp->bt_ioerror_rl, 30 * HZ,
                             DEFAULT_RATELIMIT_BURST);

        if (xfs_setsize_buftarg_early(btp, bdev))
                goto error_free;

        if (list_lru_init(&btp->bt_lru))
                goto error_free;

        if (percpu_counter_init(&btp->bt_io_count, 0, GFP_KERNEL))
                goto error_lru;

        btp->bt_shrinker.count_objects = xfs_buftarg_shrink_count;
        btp->bt_shrinker.scan_objects = xfs_buftarg_shrink_scan;
        btp->bt_shrinker.seeks = DEFAULT_SEEKS;
        btp->bt_shrinker.flags = SHRINKER_NUMA_AWARE;
        if (register_shrinker(&btp->bt_shrinker))
                goto error_pcpu;
        return btp;

error_pcpu:
        percpu_counter_destroy(&btp->bt_io_count);
error_lru:
        list_lru_destroy(&btp->bt_lru);
error_free:
        kmem_free(btp);
        return NULL;
}

/*
 * Cancel a delayed write list.
 *
 * Remove each buffer from the list, clear the delwri queue flag and drop the
 * associated buffer reference.
 */
void
xfs_buf_delwri_cancel(
        struct list_head        *list)
{
        struct xfs_buf          *bp;

        while (!list_empty(list)) {
                bp = list_first_entry(list, struct xfs_buf, b_list);

                xfs_buf_lock(bp);
                bp->b_flags &= ~_XBF_DELWRI_Q;
                list_del_init(&bp->b_list);
                xfs_buf_relse(bp);
        }
}

/*
 * Add a buffer to the delayed write list.
 *
 * This queues a buffer for writeout if it hasn't already been.  Note that
 * neither this routine nor the buffer list submission functions perform
 * any internal synchronization.  It is expected that the lists are thread-local
 * to the callers.
 *
 * Returns true if we queued up the buffer, or false if it already had
 * been on the buffer list.
 */
bool
xfs_buf_delwri_queue(
        struct xfs_buf          *bp,
        struct list_head        *list)
{
        ASSERT(xfs_buf_islocked(bp));
        ASSERT(!(bp->b_flags & XBF_READ));

        /*
         * If the buffer is already marked delwri it already is queued up
         * by someone else for imediate writeout.  Just ignore it in that
         * case.
         */
        if (bp->b_flags & _XBF_DELWRI_Q) {
                trace_xfs_buf_delwri_queued(bp, _RET_IP_);
                return false;
        }

        trace_xfs_buf_delwri_queue(bp, _RET_IP_);

        /*
         * If a buffer gets written out synchronously or marked stale while it
         * is on a delwri list we lazily remove it. To do this, the other party
         * clears the  _XBF_DELWRI_Q flag but otherwise leaves the buffer alone.
         * It remains referenced and on the list.  In a rare corner case it
         * might get readded to a delwri list after the synchronous writeout, in
         * which case we need just need to re-add the flag here.
         */
        bp->b_flags |= _XBF_DELWRI_Q;
        if (list_empty(&bp->b_list)) {
                atomic_inc(&bp->b_hold);
                list_add_tail(&bp->b_list, list);
        }

        return true;
}

/*
 * Compare function is more complex than it needs to be because
 * the return value is only 32 bits and we are doing comparisons
 * on 64 bit values
 */
static int
xfs_buf_cmp(
        void            *priv,
        struct list_head *a,
        struct list_head *b)
{
        struct xfs_buf  *ap = container_of(a, struct xfs_buf, b_list);
        struct xfs_buf  *bp = container_of(b, struct xfs_buf, b_list);
        xfs_daddr_t             diff;

        diff = ap->b_maps[0].bm_bn - bp->b_maps[0].bm_bn;
        if (diff < 0)
                return -1;
        if (diff > 0)
                return 1;
        return 0;
}

/*
 * Submit buffers for write. If wait_list is specified, the buffers are
 * submitted using sync I/O and placed on the wait list such that the caller can
 * iowait each buffer. Otherwise async I/O is used and the buffers are released
 * at I/O completion time. In either case, buffers remain locked until I/O
 * completes and the buffer is released from the queue.
 */
static int
xfs_buf_delwri_submit_buffers(
        struct list_head        *buffer_list,
        struct list_head        *wait_list)
{
        struct xfs_buf          *bp, *n;
        int                     pinned = 0;
        struct blk_plug         plug;

        list_sort(NULL, buffer_list, xfs_buf_cmp);

        blk_start_plug(&plug);
        list_for_each_entry_safe(bp, n, buffer_list, b_list) {
                if (!wait_list) {
                        if (xfs_buf_ispinned(bp)) {
                                pinned++;
                                continue;
                        }
                        if (!xfs_buf_trylock(bp))
                                continue;
                } else {
                        xfs_buf_lock(bp);
                }

                /*
                 * Someone else might have written the buffer synchronously or
                 * marked it stale in the meantime.  In that case only the
                 * _XBF_DELWRI_Q flag got cleared, and we have to drop the
                 * reference and remove it from the list here.
                 */
                if (!(bp->b_flags & _XBF_DELWRI_Q)) {
                        list_del_init(&bp->b_list);
                        xfs_buf_relse(bp);
                        continue;
                }

                trace_xfs_buf_delwri_split(bp, _RET_IP_);

                /*
                 * If we have a wait list, each buffer (and associated delwri
                 * queue reference) transfers to it and is submitted
                 * synchronously. Otherwise, drop the buffer from the delwri
                 * queue and submit async.
                 */
                bp->b_flags &= ~_XBF_DELWRI_Q;
                bp->b_flags |= XBF_WRITE;
                if (wait_list) {
                        bp->b_flags &= ~XBF_ASYNC;
                        list_move_tail(&bp->b_list, wait_list);
                } else {
                        bp->b_flags |= XBF_ASYNC;
                        list_del_init(&bp->b_list);
                }
                __xfs_buf_submit(bp, false);
        }
        blk_finish_plug(&plug);

        return pinned;
}

/*
 * Write out a buffer list asynchronously.
 *
 * This will take the @buffer_list, write all non-locked and non-pinned buffers
 * out and not wait for I/O completion on any of the buffers.  This interface
 * is only safely useable for callers that can track I/O completion by higher
 * level means, e.g. AIL pushing as the @buffer_list is consumed in this
 * function.
 *
 * Note: this function will skip buffers it would block on, and in doing so
 * leaves them on @buffer_list so they can be retried on a later pass. As such,
 * it is up to the caller to ensure that the buffer list is fully submitted or
 * cancelled appropriately when they are finished with the list. Failure to
 * cancel or resubmit the list until it is empty will result in leaked buffers
 * at unmount time.
 */
int
xfs_buf_delwri_submit_nowait(
        struct list_head        *buffer_list)
{
        return xfs_buf_delwri_submit_buffers(buffer_list, NULL);
}

/*
 * Write out a buffer list synchronously.
 *
 * This will take the @buffer_list, write all buffers out and wait for I/O
 * completion on all of the buffers. @buffer_list is consumed by the function,
 * so callers must have some other way of tracking buffers if they require such
 * functionality.
 */
int
xfs_buf_delwri_submit(
        struct list_head        *buffer_list)
{
        LIST_HEAD               (wait_list);
        int                     error = 0, error2;
        struct xfs_buf          *bp;

        xfs_buf_delwri_submit_buffers(buffer_list, &wait_list);

        /* Wait for IO to complete. */
        while (!list_empty(&wait_list)) {
                bp = list_first_entry(&wait_list, struct xfs_buf, b_list);

                list_del_init(&bp->b_list);

                /*
                 * Wait on the locked buffer, check for errors and unlock and
                 * release the delwri queue reference.
                 */
                error2 = xfs_buf_iowait(bp);
                xfs_buf_relse(bp);
                if (!error)
                        error = error2;
        }

        return error;
}

/*
 * Push a single buffer on a delwri queue.
 *
 * The purpose of this function is to submit a single buffer of a delwri queue
 * and return with the buffer still on the original queue. The waiting delwri
 * buffer submission infrastructure guarantees transfer of the delwri queue
 * buffer reference to a temporary wait list. We reuse this infrastructure to
 * transfer the buffer back to the original queue.
 *
 * Note the buffer transitions from the queued state, to the submitted and wait
 * listed state and back to the queued state during this call. The buffer
 * locking and queue management logic between _delwri_pushbuf() and
 * _delwri_queue() guarantee that the buffer cannot be queued to another list
 * before returning.
 */
int
xfs_buf_delwri_pushbuf(
        struct xfs_buf          *bp,
        struct list_head        *buffer_list)
{
        LIST_HEAD               (submit_list);
        int                     error;

        ASSERT(bp->b_flags & _XBF_DELWRI_Q);

        trace_xfs_buf_delwri_pushbuf(bp, _RET_IP_);

        /*
         * Isolate the buffer to a new local list so we can submit it for I/O
         * independently from the rest of the original list.
         */
        xfs_buf_lock(bp);
        list_move(&bp->b_list, &submit_list);
        xfs_buf_unlock(bp);

        /*
         * Delwri submission clears the DELWRI_Q buffer flag and returns with
         * the buffer on the wait list with the original reference. Rather than
         * bounce the buffer from a local wait list back to the original list
         * after I/O completion, reuse the original list as the wait list.
         */
        xfs_buf_delwri_submit_buffers(&submit_list, buffer_list);

        /*
         * The buffer is now locked, under I/O and wait listed on the original
         * delwri queue. Wait for I/O completion, restore the DELWRI_Q flag and
         * return with the buffer unlocked and on the original queue.
         */
        error = xfs_buf_iowait(bp);
        bp->b_flags |= _XBF_DELWRI_Q;
        xfs_buf_unlock(bp);

        return error;
}

int __init
xfs_buf_init(void)
{
        xfs_buf_zone = kmem_cache_create("xfs_buf", sizeof(struct xfs_buf), 0,
                                         SLAB_HWCACHE_ALIGN |
                                         SLAB_RECLAIM_ACCOUNT |
                                         SLAB_MEM_SPREAD,
                                         NULL);
        if (!xfs_buf_zone)
                goto out;

        return 0;

 out:
        return -ENOMEM;
}

void
xfs_buf_terminate(void)
{
        kmem_cache_destroy(xfs_buf_zone);
}

void xfs_buf_set_ref(struct xfs_buf *bp, int lru_ref)
{
        /*
         * Set the lru reference count to 0 based on the error injection tag.
         * This allows userspace to disrupt buffer caching for debug/testing
         * purposes.
         */
        if (XFS_TEST_ERROR(false, bp->b_mount, XFS_ERRTAG_BUF_LRU_REF))
                lru_ref = 0;

        atomic_set(&bp->b_lru_ref, lru_ref);
}

/*
 * Verify an on-disk magic value against the magic value specified in the
 * verifier structure. The verifier magic is in disk byte order so the caller is
 * expected to pass the value directly from disk.
 */
bool
xfs_verify_magic(
        struct xfs_buf          *bp,
        __be32                  dmagic)
{
        struct xfs_mount        *mp = bp->b_mount;
        int                     idx;

        idx = xfs_sb_version_hascrc(&mp->m_sb);
        if (WARN_ON(!bp->b_ops || !bp->b_ops->magic[idx]))
                return false;
        return dmagic == bp->b_ops->magic[idx];
}
/*
 * Verify an on-disk magic value against the magic value specified in the
 * verifier structure. The verifier magic is in disk byte order so the caller is
 * expected to pass the value directly from disk.
 */
bool
xfs_verify_magic16(
        struct xfs_buf          *bp,
        __be16                  dmagic)
{
        struct xfs_mount        *mp = bp->b_mount;
        int                     idx;

        idx = xfs_sb_version_hascrc(&mp->m_sb);
        if (WARN_ON(!bp->b_ops || !bp->b_ops->magic16[idx]))
                return false;
        return dmagic == bp->b_ops->magic16[idx];
}