1 files changed, 57 insertions, 8 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index fa19f2f68c1b..11aa8f743b90 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2805,7 +2805,8 @@ static int submit_extent_page(int rw, struct extent_io_tree *tree,
                               bio_end_io_t end_io_func,
                               int mirror_num,
                               unsigned long prev_bio_flags,
-                              unsigned long bio_flags)
+                              unsigned long bio_flags,
+                              bool force_bio_submit)
 {
         int ret = 0;
         struct bio *bio;
@@ -2823,6 +2824,7 @@ static int submit_extent_page(int rw, struct extent_io_tree *tree,
                         contig = bio_end_sector(bio) == sector;
 
                 if (prev_bio_flags != bio_flags || !contig ||
+                    force_bio_submit ||
                     merge_bio(rw, tree, page, offset, page_size, bio, bio_flags) ||
                     bio_add_page(bio, page, page_size, offset) < page_size) {
                         ret = submit_one_bio(rw, bio, mirror_num,
@@ -2922,7 +2924,8 @@ static int __do_readpage(struct extent_io_tree *tree,
                          get_extent_t *get_extent,
                          struct extent_map **em_cached,
                          struct bio **bio, int mirror_num,
-                         unsigned long *bio_flags, int rw)
+                         unsigned long *bio_flags, int rw,
+                         u64 *prev_em_start)
 {
         struct inode *inode = page->mapping->host;
         u64 start = page_offset(page);
@@ -2970,6 +2973,7 @@ static int __do_readpage(struct extent_io_tree *tree,
         }
         while (cur <= end) {
                 unsigned long pnr = (last_byte >> PAGE_CACHE_SHIFT) + 1;
+                bool force_bio_submit = false;
 
                 if (cur >= last_byte) {
                         char *userpage;
@@ -3020,6 +3024,49 @@ static int __do_readpage(struct extent_io_tree *tree,
                 block_start = em->block_start;
                 if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
                         block_start = EXTENT_MAP_HOLE;
+
+                /*
+                 * If we have a file range that points to a compressed extent
+                 * and it's followed by a consecutive file range that points to
+                 * to the same compressed extent (possibly with a different
+                 * offset and/or length, so it either points to the whole extent
+                 * or only part of it), we must make sure we do not submit a
+                 * single bio to populate the pages for the 2 ranges because
+                 * this makes the compressed extent read zero out the pages
+                 * belonging to the 2nd range. Imagine the following scenario:
+                 *
+                 *  File layout
+                 *  [0 - 8K]                     [8K - 24K]
+                 *    |                               |
+                 *    |                               |
+                 * points to extent X,         points to extent X,
+                 * offset 4K, length of 8K     offset 0, length 16K
+                 *
+                 * [extent X, compressed length = 4K uncompressed length = 16K]
+                 *
+                 * If the bio to read the compressed extent covers both ranges,
+                 * it will decompress extent X into the pages belonging to the
+                 * first range and then it will stop, zeroing out the remaining
+                 * pages that belong to the other range that points to extent X.
+                 * So here we make sure we submit 2 bios, one for the first
+                 * range and another one for the third range. Both will target
+                 * the same physical extent from disk, but we can't currently
+                 * make the compressed bio endio callback populate the pages
+                 * for both ranges because each compressed bio is tightly
+                 * coupled with a single extent map, and each range can have
+                 * an extent map with a different offset value relative to the
+                 * uncompressed data of our extent and different lengths. This
+                 * is a corner case so we prioritize correctness over
+                 * non-optimal behavior (submitting 2 bios for the same extent).
+                 */
+                if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags) &&
+                    prev_em_start && *prev_em_start != (u64)-1 &&
+                    *prev_em_start != em->orig_start)
+                        force_bio_submit = true;
+
+                if (prev_em_start)
+                        *prev_em_start = em->orig_start;
+
                 free_extent_map(em);
                 em = NULL;
 
@@ -3069,7 +3116,8 @@ static int __do_readpage(struct extent_io_tree *tree,
                                          bdev, bio, pnr,
                                          end_bio_extent_readpage, mirror_num,
                                          *bio_flags,
-                                         this_bio_flag);
+                                         this_bio_flag,
+                                         force_bio_submit);
                 if (!ret) {
                         nr++;
                         *bio_flags = this_bio_flag;
@@ -3101,6 +3149,7 @@ static inline void __do_contiguous_readpages(struct extent_io_tree *tree,
         struct inode *inode;
         struct btrfs_ordered_extent *ordered;
         int index;
+        u64 prev_em_start = (u64)-1;
 
         inode = pages[0]->mapping->host;
         while (1) {
@@ -3116,7 +3165,7 @@ static inline void __do_contiguous_readpages(struct extent_io_tree *tree,
 
         for (index = 0; index < nr_pages; index++) {
                 __do_readpage(tree, pages[index], get_extent, em_cached, bio,
-                              mirror_num, bio_flags, rw);
+                              mirror_num, bio_flags, rw, &prev_em_start);
                 page_cache_release(pages[index]);
         }
 }
@@ -3184,7 +3233,7 @@ static int __extent_read_full_page(struct extent_io_tree *tree,
         }
 
         ret = __do_readpage(tree, page, get_extent, NULL, bio, mirror_num,
-                            bio_flags, rw);
+                            bio_flags, rw, NULL);
         return ret;
 }
 
@@ -3210,7 +3259,7 @@ int extent_read_full_page_nolock(struct extent_io_tree *tree, struct page *page,
         int ret;
 
         ret = __do_readpage(tree, page, get_extent, NULL, &bio, mirror_num,
-                                      &bio_flags, READ);
+                            &bio_flags, READ, NULL);
         if (bio)
                 ret = submit_one_bio(READ, bio, mirror_num, bio_flags);
         return ret;
@@ -3463,7 +3512,7 @@ static noinline_for_stack int __extent_writepage_io(struct inode *inode,
                                                  sector, iosize, pg_offset,
                                                  bdev, &epd->bio, max_nr,
                                                  end_bio_extent_writepage,
-                                                 0, 0, 0);
+                                                 0, 0, 0, false);
                         if (ret)
                                 SetPageError(page);
                 }
@@ -3765,7 +3814,7 @@ static noinline_for_stack int write_one_eb(struct extent_buffer *eb,
                 ret = submit_extent_page(rw, tree, wbc, p, offset >> 9,
                                          PAGE_CACHE_SIZE, 0, bdev, &epd->bio,
                                          -1, end_bio_extent_buffer_writepage,
-                                         0, epd->bio_flags, bio_flags);
+                                         0, epd->bio_flags, bio_flags, false);
                 epd->bio_flags = bio_flags;
                 if (ret) {
                         set_btree_ioerr(p);