erofs: get rid of magical Z_EROFS_MAPPING_STAGING

Previously, we played around with magical page->mapping for short-lived
temporary pages since we need to identify different types of pages in
the same pcluster but both invalidated and short-lived temporary pages
can have page->mapping == NULL. It was considered as safe because that
temporary pages are all non-LRU / non-movable pages.

This patch tends to use specific page->private to identify short-lived
pages instead so it won't rely on page->mapping anymore. Details are
described in "compress.h" as well.

Link: https://lore.kernel.org/r/20201208095834.3133565-1-hsiangkao@redhat.com
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

diff --git a/fs/erofs/compress.h b/fs/erofs/compress.h
index 3d45244..4dadde1 100644 (file)
--- a/fs/erofs/compress.h
+++ b/fs/erofs/compress.h
@@ -26,30 +26,55 @@ struct z_erofs_decompress_req {
        bool inplace_io, partial_decoding;
 };
 
+/* some special page->private (unsigned long, see below) */
+#define Z_EROFS_SHORTLIVED_PAGE                (-1UL << 2)
+
 /*
- * - 0x5A110C8D ('sallocated', Z_EROFS_MAPPING_STAGING) -
- * used to mark temporary allocated pages from other
- * file/cached pages and NULL mapping pages.
+ * For all pages in a pcluster, page->private should be one of
+ * Type                         Last 2bits      page->private
+ * short-lived page             00              Z_EROFS_SHORTLIVED_PAGE
+ * cached/managed page          00              pointer to z_erofs_pcluster
+ * online page (file-backed,    01/10/11        sub-index << 2 | count
+ *              some pages can be used for inplace I/O)
+ *
+ * page->mapping should be one of
+ * Type                 page->mapping
+ * short-lived page     NULL
+ * cached/managed page  non-NULL or NULL (invalidated/truncated page)
+ * online page          non-NULL
+ *
+ * For all managed pages, PG_private should be set with 1 extra refcount,
+ * which is used for page reclaim / migration.
  */
-#define Z_EROFS_MAPPING_STAGING         ((void *)0x5A110C8D)
 
-/* check if a page is marked as staging */
-static inline bool z_erofs_page_is_staging(struct page *page)
+/*
+ * short-lived pages are pages directly from buddy system with specific
+ * page->private (no need to set PagePrivate since these are non-LRU /
+ * non-movable pages and bypass reclaim / migration code).
+ */
+static inline bool z_erofs_is_shortlived_page(struct page *page)
 {
-       return page->mapping == Z_EROFS_MAPPING_STAGING;
+       if (page->private != Z_EROFS_SHORTLIVED_PAGE)
+               return false;
+
+       DBG_BUGON(page->mapping);
+       return true;
 }
 
-static inline bool z_erofs_put_stagingpage(struct list_head *pagepool,
-                                          struct page *page)
+static inline bool z_erofs_put_shortlivedpage(struct list_head *pagepool,
+                                             struct page *page)
 {
-       if (!z_erofs_page_is_staging(page))
+       if (!z_erofs_is_shortlived_page(page))
                return false;
 
-       /* staging pages should not be used by others at the same time */
-       if (page_ref_count(page) > 1)
+       /* short-lived pages should not be used by others at the same time */
+       if (page_ref_count(page) > 1) {
                put_page(page);
-       else
+       } else {
+               /* follow the pcluster rule above. */
+               set_page_private(page, 0);
                list_add(&page->lru, pagepool);
+       }
        return true;
 }
 

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index cbadbf5..1cb1ffd 100644 (file)
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -76,7 +76,7 @@ static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
                        victim = erofs_allocpage(pagepool, GFP_KERNEL);
                        if (!victim)
                                return -ENOMEM;
-                       victim->mapping = Z_EROFS_MAPPING_STAGING;
+                       set_page_private(victim, Z_EROFS_SHORTLIVED_PAGE);
                }
                rq->out[i] = victim;
        }

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 86fd3bf..cfb0d11 100644 (file)
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -226,11 +226,8 @@ int erofs_try_to_free_all_cached_pages(struct erofs_sb_info *sbi,
 
                /* barrier is implied in the following 'unlock_page' */
                WRITE_ONCE(pcl->compressed_pages[i], NULL);
-               set_page_private(page, 0);
-               ClearPagePrivate(page);
-
+               detach_page_private(page);
                unlock_page(page);
-               put_page(page);
        }
        return 0;
 }
@@ -254,10 +251,8 @@ int erofs_try_to_free_cached_page(struct address_space *mapping,
                }
                erofs_workgroup_unfreeze(&pcl->obj, 1);
 
-               if (ret) {
-                       ClearPagePrivate(page);
-                       put_page(page);
-               }
+               if (ret)
+                       detach_page_private(page);
        }
        return ret;
 }
@@ -648,12 +643,12 @@ hitted:
 
 retry:
        err = z_erofs_attach_page(clt, page, page_type);
-       /* should allocate an additional staging page for pagevec */
+       /* should allocate an additional short-lived page for pagevec */
        if (err == -EAGAIN) {
                struct page *const newpage =
                                alloc_page(GFP_NOFS | __GFP_NOFAIL);
 
-               newpage->mapping = Z_EROFS_MAPPING_STAGING;
+               set_page_private(newpage, Z_EROFS_SHORTLIVED_PAGE);
                err = z_erofs_attach_page(clt, newpage,
                                          Z_EROFS_PAGE_TYPE_EXCLUSIVE);
                if (!err)
@@ -710,6 +705,11 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
                queue_work(z_erofs_workqueue, &io->u.work);
 }
 
+static bool z_erofs_page_is_invalidated(struct page *page)
+{
+       return !page->mapping && !z_erofs_is_shortlived_page(page);
+}
+
 static void z_erofs_decompressqueue_endio(struct bio *bio)
 {
        tagptr1_t t = tagptr_init(tagptr1_t, bio->bi_private);
@@ -722,7 +722,7 @@ static void z_erofs_decompressqueue_endio(struct bio *bio)
                struct page *page = bvec->bv_page;
 
                DBG_BUGON(PageUptodate(page));
-               DBG_BUGON(!page->mapping);
+               DBG_BUGON(z_erofs_page_is_invalidated(page));
 
                if (err)
                        SetPageError(page);
@@ -795,9 +795,9 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
 
                /* all pages in pagevec ought to be valid */
                DBG_BUGON(!page);
-               DBG_BUGON(!page->mapping);
+               DBG_BUGON(z_erofs_page_is_invalidated(page));
 
-               if (z_erofs_put_stagingpage(pagepool, page))
+               if (z_erofs_put_shortlivedpage(pagepool, page))
                        continue;
 
                if (page_type == Z_EROFS_VLE_PAGE_TYPE_HEAD)
@@ -831,9 +831,9 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
 
                /* all compressed pages ought to be valid */
                DBG_BUGON(!page);
-               DBG_BUGON(!page->mapping);
+               DBG_BUGON(z_erofs_page_is_invalidated(page));
 
-               if (!z_erofs_page_is_staging(page)) {
+               if (!z_erofs_is_shortlived_page(page)) {
                        if (erofs_page_is_managed(sbi, page)) {
                                if (!PageUptodate(page))
                                        err = -EIO;
@@ -858,7 +858,7 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
                        overlapped = true;
                }
 
-               /* PG_error needs checking for inplaced and staging pages */
+               /* PG_error needs checking for all non-managed pages */
                if (PageError(page)) {
                        DBG_BUGON(PageUptodate(page));
                        err = -EIO;
@@ -897,8 +897,8 @@ out:
                if (erofs_page_is_managed(sbi, page))
                        continue;
 
-               /* recycle all individual staging pages */
-               (void)z_erofs_put_stagingpage(pagepool, page);
+               /* recycle all individual short-lived pages */
+               (void)z_erofs_put_shortlivedpage(pagepool, page);
 
                WRITE_ONCE(compressed_pages[i], NULL);
        }
@@ -908,10 +908,10 @@ out:
                if (!page)
                        continue;
 
-               DBG_BUGON(!page->mapping);
+               DBG_BUGON(z_erofs_page_is_invalidated(page));
 
-               /* recycle all individual staging pages */
-               if (z_erofs_put_stagingpage(pagepool, page))
+               /* recycle all individual short-lived pages */
+               if (z_erofs_put_shortlivedpage(pagepool, page))
                        continue;
 
                if (err < 0)
@@ -1011,13 +1011,17 @@ repeat:
        mapping = READ_ONCE(page->mapping);
 
        /*
-        * unmanaged (file) pages are all locked solidly,
+        * file-backed online pages in plcuster are all locked steady,
         * therefore it is impossible for `mapping' to be NULL.
         */
        if (mapping && mapping != mc)
                /* ought to be unmanaged pages */
                goto out;
 
+       /* directly return for shortlived page as well */
+       if (z_erofs_is_shortlived_page(page))
+               goto out;
+
        lock_page(page);
 
        /* only true if page reclaim goes wrong, should never happen */
@@ -1062,8 +1066,8 @@ repeat:
 out_allocpage:
        page = erofs_allocpage(pagepool, gfp | __GFP_NOFAIL);
        if (!tocache || add_to_page_cache_lru(page, mc, index + nr, gfp)) {
-               /* non-LRU / non-movable temporary page is needed */
-               page->mapping = Z_EROFS_MAPPING_STAGING;
+               /* turn into temporary page if fails */
+               set_page_private(page, Z_EROFS_SHORTLIVED_PAGE);
                tocache = false;
        }
 
@@ -1080,8 +1084,9 @@ out_allocpage:
        }
 
        if (tocache) {
-               set_page_private(page, (unsigned long)pcl);
-               SetPagePrivate(page);
+               attach_page_private(page, pcl);
+               /* drop a ref added by allocpage (then we have 2 refs here) */
+               put_page(page);
        }
 out:   /* the only exit (for tracing and debugging) */
        return page;

diff --git a/fs/erofs/zdata.h b/fs/erofs/zdata.h
index 68c9b29..b503b35 100644 (file)
--- a/fs/erofs/zdata.h
+++ b/fs/erofs/zdata.h
@@ -173,6 +173,7 @@ static inline void z_erofs_onlinepage_endio(struct page *page)
 
        v = atomic_dec_return(u.o);
        if (!(v & Z_EROFS_ONLINEPAGE_COUNT_MASK)) {
+               set_page_private(page, 0);
                ClearPagePrivate(page);
                if (!PageError(page))
                        SetPageUptodate(page);