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

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

[robbieko]

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

[Filipe Manana]

On Fri, May 8, 2026 at 2:50 PM robbieko <robbieko@synology.com> wrote:
>
> 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>

As stated before, don't leave a blank line between the Fixes and SOB tags.

> ---

Following the ---, please describe the changes between v2 and v1 in the future.

I've added the patch to the for-next branch on github, with minor
changes to the change log.
Thanks.

Reviewed-by: Filipe Manana <fdmanana@suse.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
>
>
> Disclaimer: The contents of this e-mail message and any attachments are confidential and are intended solely for addressee. The information may also be legally privileged. This transmission is sent in trust, for the sole purpose of delivery to the intended recipient. If you have received this transmission in error, any use, reproduction or dissemination of this transmission is strictly prohibited. If you are not the intended recipient, please immediately notify the sender by reply e-mail or phone and delete this message and its attachments, if any.
>

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

[Christoph Hellwig]

On Fri, May 08, 2026 at 09:42:11PM +0800, robbieko wrote:
> The following reproducer triggers the problem:

Please add this to xfstests.