Btrfs: fix defrag regression

If a file has 3 small extents:

| ext1 | ext2 | ext3 |

Running "btrfs fi defrag" will only defrag the last two extents, if those
extent mappings hasn't been read into memory from disk.

This bug was introduced by commit 17ce6ef8d731af5edac8c39e806db4c7e1f6956f
("Btrfs: add a check to decide if we should defrag the range")

The cause is, that commit looked into previous and next extents using
lookup_extent_mapping() only.

While at it, remove the code that checks the previous extent, since
it's sufficient to check the next extent.

Signed-off-by: Li Zefan <lizefan@huawei.com>

1 files changed, 49 insertions, 48 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index c5254dde1f0f..a98f7d252829 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -786,39 +786,57 @@ none:
         return -ENOENT;
 }
 
-/*
- * Validaty check of prev em and next em:
- * 1) no prev/next em
- * 2) prev/next em is an hole/inline extent
- */
-static int check_adjacent_extents(struct inode *inode, struct extent_map *em)
+static struct extent_map *defrag_lookup_extent(struct inode *inode, u64 start)
 {
         struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
-        struct extent_map *prev = NULL, *next = NULL;
-        int ret = 0;
+        struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
+        struct extent_map *em;
+        u64 len = PAGE_CACHE_SIZE;
 
+        /*
+         * hopefully we have this extent in the tree already, try without
+         * the full extent lock
+         */
         read_lock(&em_tree->lock);
-        prev = lookup_extent_mapping(em_tree, em->start - 1, (u64)-1);
-        next = lookup_extent_mapping(em_tree, em->start + em->len, (u64)-1);
+        em = lookup_extent_mapping(em_tree, start, len);
         read_unlock(&em_tree->lock);
 
-        if ((!prev || prev->block_start >= EXTENT_MAP_LAST_BYTE) &&
-            (!next || next->block_start >= EXTENT_MAP_LAST_BYTE))
-                ret = 1;
-        free_extent_map(prev);
-        free_extent_map(next);
+        if (!em) {
+                /* get the big lock and read metadata off disk */
+                lock_extent(io_tree, start, start + len - 1);
+                em = btrfs_get_extent(inode, NULL, 0, start, len, 0);
+                unlock_extent(io_tree, start, start + len - 1);
 
+                if (IS_ERR(em))
+                        return NULL;
+        }
+
+        return em;
+}
+
+static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em)
+{
+        struct extent_map *next;
+        bool ret = true;
+
+        /* this is the last extent */
+        if (em->start + em->len >= i_size_read(inode))
+                return false;
+
+        next = defrag_lookup_extent(inode, em->start + em->len);
+        if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE)
+                ret = false;
+
+        free_extent_map(next);
         return ret;
 }
 
-static int should_defrag_range(struct inode *inode, u64 start, u64 len,
-                               int thresh, u64 *last_len, u64 *skip,
-                               u64 *defrag_end)
+static int should_defrag_range(struct inode *inode, u64 start, int thresh,
+                               u64 *last_len, u64 *skip, u64 *defrag_end)
 {
-        struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
-        struct extent_map *em = NULL;
-        struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
+        struct extent_map *em;
         int ret = 1;
+        bool next_mergeable = true;
 
         /*
          * make sure that once we start defragging an extent, we keep on
@@ -829,23 +847,9 @@ static int should_defrag_range(struct inode *inode, u64 start, u64 len,
 
         *skip = 0;
 
-        /*
-         * hopefully we have this extent in the tree already, try without
-         * the full extent lock
-         */
-        read_lock(&em_tree->lock);
-        em = lookup_extent_mapping(em_tree, start, len);
-        read_unlock(&em_tree->lock);
-
-        if (!em) {
-                /* get the big lock and read metadata off disk */
-                lock_extent(io_tree, start, start + len - 1);
-                em = btrfs_get_extent(inode, NULL, 0, start, len, 0);
-                unlock_extent(io_tree, start, start + len - 1);
-
-                if (IS_ERR(em))
-                        return 0;
-        }
+        em = defrag_lookup_extent(inode, start);
+        if (!em)
+                return 0;
 
         /* this will cover holes, and inline extents */
         if (em->block_start >= EXTENT_MAP_LAST_BYTE) {
@@ -853,18 +857,15 @@ static int should_defrag_range(struct inode *inode, u64 start, u64 len,
                 goto out;
         }
 
-        /* If we have nothing to merge with us, just skip. */
-        if (check_adjacent_extents(inode, em)) {
-                ret = 0;
-                goto out;
-        }
+        next_mergeable = defrag_check_next_extent(inode, em);
 
         /*
-         * we hit a real extent, if it is big don't bother defragging it again
+         * we hit a real extent, if it is big or the next extent is not a
+         * real extent, don't bother defragging it
          */
-        if ((*last_len == 0 || *last_len >= thresh) && em->len >= thresh)
+        if ((*last_len == 0 || *last_len >= thresh) &&
+            (em->len >= thresh || !next_mergeable))
                 ret = 0;
-
 out:
         /*
          * last_len ends up being a counter of how many bytes we've defragged.
@@ -1143,8 +1144,8 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
                         break;
 
                 if (!should_defrag_range(inode, (u64)i << PAGE_CACHE_SHIFT,
-                                         PAGE_CACHE_SIZE, extent_thresh,
-                                         &last_len, &skip, &defrag_end)) {
+                                         extent_thresh, &last_len, &skip,
+                                         &defrag_end)) {
                         unsigned long next;
                         /*
                          * the should_defrag function tells us how much to skip