userfaultfd/shmem: support UFFDIO_CONTINUE for shmem

With this change, userspace can resolve a minor fault within a
shmem-backed area with a UFFDIO_CONTINUE ioctl.  The semantics for this
match those for hugetlbfs - we look up the existing page in the page
cache, and install a PTE for it.

This commit introduces a new helper: mfill_atomic_install_pte.

Why handle UFFDIO_CONTINUE for shmem in mm/userfaultfd.c, instead of in
shmem.c?  The existing userfault implementation only relies on shmem.c for
VM_SHARED VMAs.  However, minor fault handling / CONTINUE work just fine
for !VM_SHARED VMAs as well.  We'd prefer to handle CONTINUE for shmem in
one place, regardless of shared/private (to reduce code duplication).

Why add a new mfill_atomic_install_pte helper?  A problem we have with
continue is that shmem_mfill_atomic_pte() and mcopy_atomic_pte() are
*close* to what we want, but not exactly.  We do want to setup the PTEs in
a CONTINUE operation, but we don't want to e.g.  allocate a new page,
charge it (e.g.  to the shmem inode), manipulate various flags, etc.  Also
we have the problem stated above: shmem_mfill_atomic_pte() and
mcopy_atomic_pte() both handle one-half of the problem (shared / private)
continue cares about.  So, introduce mcontinue_atomic_pte(), to handle all
of the shmem continue cases.  Introduce the helper so it doesn't duplicate
code with mcopy_atomic_pte().

In a future commit, shmem_mfill_atomic_pte() will also be modified to use
this new helper.  However, since this is a bigger refactor, it seems most
clear to do it as a separate change.

Link: https://lkml.kernel.org/r/20210503180737.2487560-5-axelrasmussen@google.com
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Joe Perches <joe@perches.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Wang Qing <wangqing@vivo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 57b228b..218ec2a 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -48,6 +48,83 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
        return dst_vma;
 }
 
+/*
+ * Install PTEs, to map dst_addr (within dst_vma) to page.
+ *
+ * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
+ * whether or not dst_vma is VM_SHARED. It also handles the more general
+ * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
+ * backed, or not).
+ *
+ * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
+ * shmem_mcopy_atomic_pte instead.
+ */
+static int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
+                                   struct vm_area_struct *dst_vma,
+                                   unsigned long dst_addr, struct page *page,
+                                   bool newly_allocated, bool wp_copy)
+{
+       int ret;
+       pte_t _dst_pte, *dst_pte;
+       bool writable = dst_vma->vm_flags & VM_WRITE;
+       bool vm_shared = dst_vma->vm_flags & VM_SHARED;
+       bool page_in_cache = page->mapping;
+       spinlock_t *ptl;
+       struct inode *inode;
+       pgoff_t offset, max_off;
+
+       _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
+       if (page_in_cache && !vm_shared)
+               writable = false;
+       if (writable || !page_in_cache)
+               _dst_pte = pte_mkdirty(_dst_pte);
+       if (writable) {
+               if (wp_copy)
+                       _dst_pte = pte_mkuffd_wp(_dst_pte);
+               else
+                       _dst_pte = pte_mkwrite(_dst_pte);
+       }
+
+       dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+
+       if (vma_is_shmem(dst_vma)) {
+               /* serialize against truncate with the page table lock */
+               inode = dst_vma->vm_file->f_inode;
+               offset = linear_page_index(dst_vma, dst_addr);
+               max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+               ret = -EFAULT;
+               if (unlikely(offset >= max_off))
+                       goto out_unlock;
+       }
+
+       ret = -EEXIST;
+       if (!pte_none(*dst_pte))
+               goto out_unlock;
+
+       if (page_in_cache)
+               page_add_file_rmap(page, false);
+       else
+               page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
+
+       /*
+        * Must happen after rmap, as mm_counter() checks mapping (via
+        * PageAnon()), which is set by __page_set_anon_rmap().
+        */
+       inc_mm_counter(dst_mm, mm_counter(page));
+
+       if (newly_allocated)
+               lru_cache_add_inactive_or_unevictable(page, dst_vma);
+
+       set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+       /* No need to invalidate - it was non-present before */
+       update_mmu_cache(dst_vma, dst_addr, dst_pte);
+       ret = 0;
+out_unlock:
+       pte_unmap_unlock(dst_pte, ptl);
+       return ret;
+}
+
 static int mcopy_atomic_pte(struct mm_struct *dst_mm,
                            pmd_t *dst_pmd,
                            struct vm_area_struct *dst_vma,
@@ -56,13 +133,9 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
                            struct page **pagep,
                            bool wp_copy)
 {
-       pte_t _dst_pte, *dst_pte;
-       spinlock_t *ptl;
        void *page_kaddr;
        int ret;
        struct page *page;
-       pgoff_t offset, max_off;
-       struct inode *inode;
 
        if (!*pagep) {
                ret = -ENOMEM;
@@ -99,43 +172,12 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
        if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))
                goto out_release;
 
-       _dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
-       if (dst_vma->vm_flags & VM_WRITE) {
-               if (wp_copy)
-                       _dst_pte = pte_mkuffd_wp(_dst_pte);
-               else
-                       _dst_pte = pte_mkwrite(_dst_pte);
-       }
-
-       dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
-       if (dst_vma->vm_file) {
-               /* the shmem MAP_PRIVATE case requires checking the i_size */
-               inode = dst_vma->vm_file->f_inode;
-               offset = linear_page_index(dst_vma, dst_addr);
-               max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-               ret = -EFAULT;
-               if (unlikely(offset >= max_off))
-                       goto out_release_uncharge_unlock;
-       }
-       ret = -EEXIST;
-       if (!pte_none(*dst_pte))
-               goto out_release_uncharge_unlock;
-
-       inc_mm_counter(dst_mm, MM_ANONPAGES);
-       page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
-       lru_cache_add_inactive_or_unevictable(page, dst_vma);
-
-       set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
-
-       /* No need to invalidate - it was non-present before */
-       update_mmu_cache(dst_vma, dst_addr, dst_pte);
-
-       pte_unmap_unlock(dst_pte, ptl);
-       ret = 0;
+       ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
+                                      page, true, wp_copy);
+       if (ret)
+               goto out_release;
 out:
        return ret;
-out_release_uncharge_unlock:
-       pte_unmap_unlock(dst_pte, ptl);
 out_release:
        put_page(page);
        goto out;
@@ -176,6 +218,41 @@ out_unlock:
        return ret;
 }
 
+/* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
+static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
+                               pmd_t *dst_pmd,
+                               struct vm_area_struct *dst_vma,
+                               unsigned long dst_addr,
+                               bool wp_copy)
+{
+       struct inode *inode = file_inode(dst_vma->vm_file);
+       pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
+       struct page *page;
+       int ret;
+
+       ret = shmem_getpage(inode, pgoff, &page, SGP_READ);
+       if (ret)
+               goto out;
+       if (!page) {
+               ret = -EFAULT;
+               goto out;
+       }
+
+       ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
+                                      page, false, wp_copy);
+       if (ret)
+               goto out_release;
+
+       unlock_page(page);
+       ret = 0;
+out:
+       return ret;
+out_release:
+       unlock_page(page);
+       put_page(page);
+       goto out;
+}
+
 static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 {
        pgd_t *pgd;
@@ -367,11 +444,16 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
                                                unsigned long dst_addr,
                                                unsigned long src_addr,
                                                struct page **page,
-                                               bool zeropage,
+                                               enum mcopy_atomic_mode mode,
                                                bool wp_copy)
 {
        ssize_t err;
 
+       if (mode == MCOPY_ATOMIC_CONTINUE) {
+               return mcontinue_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
+                                           wp_copy);
+       }
+
        /*
         * The normal page fault path for a shmem will invoke the
         * fault, fill the hole in the file and COW it right away. The
@@ -383,7 +465,7 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
         * and not in the radix tree.
         */
        if (!(dst_vma->vm_flags & VM_SHARED)) {
-               if (!zeropage)
+               if (mode == MCOPY_ATOMIC_NORMAL)
                        err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
                                               dst_addr, src_addr, page,
                                               wp_copy);
@@ -393,7 +475,8 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
        } else {
                VM_WARN_ON_ONCE(wp_copy);
                err = shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
-                                            dst_addr, src_addr, zeropage,
+                                            dst_addr, src_addr,
+                                            mode != MCOPY_ATOMIC_NORMAL,
                                             page);
        }
 
@@ -415,7 +498,6 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
        long copied;
        struct page *page;
        bool wp_copy;
-       bool zeropage = (mcopy_mode == MCOPY_ATOMIC_ZEROPAGE);
 
        /*
         * Sanitize the command parameters:
@@ -478,7 +560,7 @@ retry:
 
        if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
                goto out_unlock;
-       if (mcopy_mode == MCOPY_ATOMIC_CONTINUE)
+       if (!vma_is_shmem(dst_vma) && mcopy_mode == MCOPY_ATOMIC_CONTINUE)
                goto out_unlock;
 
        /*
@@ -526,7 +608,7 @@ retry:
                BUG_ON(pmd_trans_huge(*dst_pmd));
 
                err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
-                                      src_addr, &page, zeropage, wp_copy);
+                                      src_addr, &page, mcopy_mode, wp_copy);
                cond_resched();
 
                if (unlikely(err == -ENOENT)) {