Re: ext4 data corruption in 6.1 stable tree (was Re: [PATCH 5.15 000/297] 5.15.140-rc1 review)

[Guenter Roeck]

On Tue, Dec 05, 2023 at 01:21:22PM +0100, Jan Kara wrote:
[ ... ]
> 
> So I've got back to this and the failure is a subtle interaction between
> iomap code and ext4 code. In particular that fact that commit 936e114a245b6
> ("iomap: update ki_pos a little later in iomap_dio_complete") is not in
> stable causes that file position is not updated after direct IO write and
> thus we direct IO writes are ending in wrong locations effectively
> corrupting data. The subtle detail is that before this commit if ->end_io
> handler returns non-zero value (which the new ext4 ->end_io handler does),
> file pos doesn't get updated, after this commit it doesn't get updated only
> if the return value is < 0.
> 
> The commit got merged in 6.5-rc1 so all stable kernels that have
> 91562895f803 ("ext4: properly sync file size update after O_SYNC direct
> IO") before 6.5 are corrupting data - I've noticed at least 6.1 is still
> carrying the problematic commit. Greg, please take out the commit from all
> stable kernels before 6.5 as soon as possible, we'll figure out proper
> backport once user data are not being corrupted anymore. Thanks!
> 

Thanks a lot for the update.

Turns out this is causing a regression in chromeos-6.1, and reverting the
offending patch fixes the problem. I suspect anyone running v6.1.64+ may
have a problem.

Guenter