[PATCH v2] btrfs: mark file extent range dirty after converting prealloc extents

[robbieko <robbieko@synology.com>]

From: Robbie Ko <robbieko@synology.com>

When writing into a preallocated extent, ordered extent completion calls
btrfs_mark_extent_written() to convert the file extent item from
BTRFS_FILE_EXTENT_PREALLOC to BTRFS_FILE_EXTENT_REG.

If the preallocated extent was created beyond i_size with fallocate
keep-size, and the inode is evicted and loaded again before the write,
the inode's file_extent_tree is initialized only up to i_size.

The out-of-i_size prealloc extent is therefore not tracked there.

After a write into that extent extends i_size, btrfs_mark_extent_written()
updates the file extent item, but the corresponding range is not marked
dirty in the inode's file_extent_tree. 

This can leave disk_i_size stale when the filesystem does not use the
no-holes feature, so after remount the file size can go back to the old
value.

The following reproducer triggers the problem:

  mkfs.btrfs -f -O ^no-holes $DEV
  mount $DEV $MNT
  touch $MNT/file
  fallocate -n -l 2M $MNT/file
  umount $MNT
  mount $DEV $MNT
  dd if=/dev/zero of=$MNT/file bs=1M count=1 conv=notrunc
  ls -l $MNT/file
  umount $MNT
  mount $DEV $MNT
  ls -l $MNT/file
  umount $MNT

Running the reproducer gives the following result:

-rw------- 1 root root 1048576 May  8 21:20 /mnt/file
-rw------- 1 root root 0 May  8 21:20 /mnt/file

Fix this by marking the written range dirty in file_extent_tree after
successfully converting the prealloc extent to a regular extent.

Fixes: 9ddc959e802b ("btrfs: use the file extent tree infrastructure")

Signed-off-by: Robbie Ko <robbieko@synology.com>
---
 fs/btrfs/file.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index cf1cb5c4db75..8c171ed07008 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -633,7 +633,7 @@ int btrfs_mark_extent_written(struct btrfs_trans_handle *trans,
 							 trans->transid);
 			btrfs_set_file_extent_num_bytes(leaf, fi,
 							end - other_start);
-			return 0;
+			goto mark_dirty;
 		}
 	}
 
@@ -661,7 +661,7 @@ int btrfs_mark_extent_written(struct btrfs_trans_handle *trans,
 							other_end - start);
 			btrfs_set_file_extent_offset(leaf, fi,
 						     start - orig_offset);
-			return 0;
+			goto mark_dirty;
 		}
 	}
 
@@ -788,7 +788,12 @@ int btrfs_mark_extent_written(struct btrfs_trans_handle *trans,
 		}
 	}
 
-	return 0;
+mark_dirty:
+	ret = btrfs_inode_set_file_extent_range(inode, start, end - start);
+	if (ret)
+		btrfs_abort_transaction(trans, ret);
+
+	return ret;
 }
 
 /*
-- 
2.43.0


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