btrfs: Continue write in case of can_not_nocow

btrfs failed in xfstests btrfs/080 with -o nodatacow.

Can be reproduced by following script:
  DEV=/dev/vdg
  MNT=/mnt/tmp

  umount $DEV &>/dev/null
  mkfs.btrfs -f $DEV
  mount -o nodatacow $DEV $MNT

  dd if=/dev/zero of=$MNT/test bs=1 count=2048 &
  btrfs subvolume snapshot -r $MNT $MNT/test_snap &
  wait
  --
  We can see dd failed on NO_SPACE.

Reason:
  __btrfs_buffered_write should run cow write when no_cow impossible,
  and current code is designed with above logic.
  But check_can_nocow() have 2 type of return value(0 and <0) on
  can_not_no_cow, and current code only continue write on first case,
  the second case happened in doing subvolume.

Fix:
  Continue write when check_can_nocow() return 0 and <0.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 098bb8f..cadfeba 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1525,27 +1525,24 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 
                reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
 
-               if (BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
-                                            BTRFS_INODE_PREALLOC)) {
-                       ret = check_can_nocow(inode, pos, &write_bytes);
-                       if (ret < 0)
-                               break;
-                       if (ret > 0) {
-                               /*
-                                * For nodata cow case, no need to reserve
-                                * data space.
-                                */
-                               only_release_metadata = true;
-                               /*
-                                * our prealloc extent may be smaller than
-                                * write_bytes, so scale down.
-                                */
-                               num_pages = DIV_ROUND_UP(write_bytes + offset,
-                                                        PAGE_CACHE_SIZE);
-                               reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
-                               goto reserve_metadata;
-                       }
+               if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
+                                             BTRFS_INODE_PREALLOC)) &&
+                   check_can_nocow(inode, pos, &write_bytes) > 0) {
+                       /*
+                        * For nodata cow case, no need to reserve
+                        * data space.
+                        */
+                       only_release_metadata = true;
+                       /*
+                        * our prealloc extent may be smaller than
+                        * write_bytes, so scale down.
+                        */
+                       num_pages = DIV_ROUND_UP(write_bytes + offset,
+                                                PAGE_CACHE_SIZE);
+                       reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
+                       goto reserve_metadata;
                }
+
                ret = btrfs_check_data_free_space(inode, pos, write_bytes);
                if (ret < 0)
                        break;