btrfs: Continue write in case of can_not_nocow

[ Upstream commit 4da2e26a ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/btrfs/file.c

@@ -1526,12 +1526,9 @@ 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) {
+		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.
@@ -1546,7 +1543,7 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 			reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
 			goto reserve_metadata;
 		}
-		}
+
 		ret = btrfs_check_data_free_space(inode, pos, write_bytes);
 		if (ret < 0)
 			break;