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

[Jens Axboe]

On 9/12/24 4:25 PM, Linus Torvalds wrote:
> On Thu, 12 Sept 2024 at 15:12, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>
>> When I saw Christian's report, I seemed to recall that we ran into this
>> at Meta too. And we did, and hence have been reverting it since our 5.19
>> release (and hence 6.4, 6.9, and 6.11 next). We should not be shipping
>> things that are known broken.
> 
> I do think that if we have big sites just reverting it as known broken
> and can't figure out why, we should do so upstream too.

Agree. I suspect it would've come up internally shortly too, as we're
just now preparing to roll 6.11 as the next kernel. That always starts
with a list of "what commits are in our 6.9 tree that aren't upstream"
and then porting those, and this one is in that (pretty short) list.

> Yes,  it's going to make it even harder to figure out what's wrong.
> Not great. But if this causes filesystem corruption, that sure isn't
> great either. And people end up going "I'll use ext4 which doesn't
> have the problem", that's not exactly helpful either.

Until someone has a good reproducer for it, it is going to remain
elusive. And it's a two-liner to enable it again for testing, hence
should not be a hard thing to do.

> And yeah, the reason ext4 doesn't have the problem is simply because
> ext4 doesn't enable large folios. So that doesn't pin anything down
> either (ie it does *not* say "this is an xfs bug" - it obviously might
> be, but it's probably more likely some large-folio issue).
> 
> Other filesystems do enable large folios (afs, bcachefs, erofs, nfs,
> smb), but maybe just not be used under the kind of load to show it.

It might be an iomap thing... Other file systems do use it, but to
various degrees, and XFS is definitely the primary user.

> 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...

Double agree, I was pretty surprised when I learned of all this today.

-- 
Jens Axboe