Re: Known and unfixed active data loss bug in MM + XFS with large folios since Dec 2021 (any kernel from 6.1 upwards)

[Dave Chinner <david@fromorbit.com>]

On Thu, Sep 12, 2024 at 03:25:50PM -0700, Linus Torvalds wrote:
> On Thu, 12 Sept 2024 at 15:12, Jens Axboe <axboe@kernel.dk> wrote:
> Honestly, the fact that it hasn't been reverted after apparently
> people knowing about it for months is a bit shocking to me. Filesystem
> people tend to take unknown corruption issues as a big deal. What
> makes this so special? Is it because the XFS people don't consider it
> an XFS issue, so...

I don't think this is a data corruption/loss problem - it certainly
hasn't ever appeared that way to me.  The "data loss" appeared to be
in incomplete postgres dump files after the system was rebooted and
this is exactly what would happen when you randomly crash the
system. i.e. dirty data in memory is lost, and application data
being written at the time is in an inconsistent state after the
system recovers. IOWs, there was no clear evidence of actual data
corruption occuring, and data loss is definitely expected when the
page cache iteration hangs and the system is forcibly rebooted
without being able to sync or unmount the filesystems...

All the hangs seem to be caused by folio lookup getting stuck
on a rogue xarray entry in truncate or readahead. If we find an
invalid entry or a folio from a different mapping or with a
unexpected index, we skip it and try again.  Hence this does not
appear to be a data corruption vector, either - it results in a
livelock from endless retry because of the bad entry in the xarray.
This endless retry livelock appears to be what is being reported.

IOWs, there is no evidence of real runtime data corruption or loss
from this pagecache livelock bug.  We also haven't heard of any
random file data corruption events since we've enabled large folios
on XFS. Hence there really is no evidence to indicate that there is
a large folio xarray lookup bug that results in data corruption in
the existing code, and therefore there is no obvious reason for
turning off the functionality we are already building significant
new functionality on top of.

It's been 10 months since I asked Christain to help isolate a
reproducer so we can track this down. Nothing came from that, so
we're still at exactly where we were at back in november 2023 -
waiting for information on a way to reproduce this issue more
reliably.

-Dave.
-- 
Dave Chinner
david@fromorbit.com