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

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Message-ID: <CAHk-=whPYGhCWOD-K2zCTwDrCK27Y0GST-nt+cb9QPzxO-iSHw@mail.gmail.com>
Date: Thu, 19 Sep 2024 06:46:46 +0200
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Jens Axboe <axboe@...nel.dk>, Dave Chinner <david@...morbit.com>, Chris Mason <clm@...a.com>, 
	Christian Theune <ct@...ingcircus.io>, linux-mm@...ck.org, 
	"linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Daniel Dao <dqminh@...udflare.com>, 
	regressions@...ts.linux.dev, regressions@...mhuis.info
Subject: Re: Known and unfixed active data loss bug in MM + XFS with large
 folios since Dec 2021 (any kernel from 6.1 upwards)

On Thu, 19 Sept 2024 at 06:36, Matthew Wilcox <willy@...radead.org> wrote:
>
> Probably xas_split_alloc() needs to just do the alloc, like the name
> says, and drop the 'entry' argument.  ICBW, but I think it explains
> what you're seeing?  Maybe it doesn't?

.. or we make the rule be that you have to re-check that the order and
the entry still matches when you do the actual xas_split()..

Like commit 6758c1128ceb does, in this case.

We do have another xas_split_alloc() - in the hugepage case - but
there we do have

                xas_lock(&xas);
                xas_reset(&xas);
                if (xas_load(&xas) != folio)
                        goto fail;

and the folio is locked over the whole sequence, so I think that code
is probably safe and guarantees that we're splitting with the same
details we alloc'ed.

                Linus