Btrfs: adjust len of writes if following a preallocated extent

If we have

|0--hole--4095||4096--preallocate--12287|

instead of using preallocated space, a 8K direct write will just
create a new 8K extent and it'll end up with

|0--new extent--8191||8192--preallocate--12287|

It's because we find a hole em and then go to create a new 8K
extent directly without adjusting @len.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Chris Mason <clm@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 06dc95caa6f17d9ad5c326d503b7e16b13d8a62f..66c1d65bd4766ad841ea07bff7dea06a041e3dc9 100644 (file)
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7782,10 +7782,12 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
        }
 
        /*
-        * this will cow the extent, reset the len in case we changed
-        * it above
+        * this will cow the extent, if em is within [start, len], then
+        * probably we've found a preallocated/existing extent, let's
+        * give it a chance to use preallocated space.
         */
-       len = bh_result->b_size;
+       len = min_t(u64, bh_result->b_size, em->len - (start - em->start));
+       len = ALIGN(len, root->sectorsize);
        free_extent_map(em);
        em = btrfs_new_extent_direct(inode, start, len);
        if (IS_ERR(em)) {