Re: [PATCH] btrfs: fix chattr failing on nodatasum files that have the compress flag

[Hengyu Liang]

On 2026/10/11 07:47, Qu Wenruo wrote:
> This is very LLM-like, and very LLM's objective-oriented behavior.
> Disclose it first.

Yes, and sorry for not saying so in the patch. I found this while
comparing the behaviour of different kernel versions. I used an LLM to
analyse the difference, write the patch and the changelog and run the
tests in VMs.

> Explain the reason why it fails.

chattr reads the flags and writes the whole set back, so FS_COMPR_FL is
in the new flags. btrfs_fileattr_set() then calls btrfs_set_prop() for
btrfs.compression, and since cf1afec09e9f that runs
prop_compression_validate(), which returns -EINVAL because
btrfs_inode_can_compress() is false for a NODATASUM inode.

> So when setting compress in the first place, the NODATASUM/NODATACOW
> flag should be cleared.

OK, I will do it this way: a new inode that inherits the compress flag
does not get NODATASUM/NODATACOW from the mount options, and chattr +c
on an empty file clears NODATASUM.

This differs from the documentation in one place. btrfs(5) says that
all files created under nodatasum have no checksums and that
compression is disabled when nodatasum is enabled. With this change a
file created in a +c directory on a nodatasum mount has checksums and
is compressed. Is that what you intend?

In chattr there are two cases where I would not clear the flag:

- The file has NODATASUM and already has data. The existing extents
  have no checksums, so clearing NODATASUM would make them unreadable.
  chattr +c would return -EINVAL here, the same as setting the
  btrfs.compression property does since commit 0e852ab8974c ("btrfs:
  do not allow compression on nodatacow files").

- The file has +C. check_fsflags() already rejects chattr +c on it
  with -EINVAL and I would keep that.

One more question before v2. Files with both flags and data can already
exist on disk, and chattr +A on them fails since 7.2. Is it OK to leave
the compress flag and the property alone on these files when only other
flags change?

Thanks,
Hengyu