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-=wgQ_OeAaNMA7A=icuf66r7Atz1-NNs9Qk8O=2gEjd=qTw@mail.gmail.com>
Date: Thu, 19 Sep 2024 05:12:59 +0200
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Dave Chinner <david@...morbit.com>
Cc: Matthew Wilcox <willy@...radead.org>, Chris Mason <clm@...a.com>, Jens Axboe <axboe@...nel.dk>, 
	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 05:03, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I think we should just do the simple one-liner of adding a
> "xas_reset()" to after doing xas_split_alloc() (or do it inside the
> xas_split_alloc()).

.. and obviously that should be actually *verified* to fix the issue
not just with the test-case that Chris and Jens have been using, but
on Christian's real PostgreSQL load.

Christian?

Note that the xas_reset() needs to be done after the check for errors
- or like Willy suggested, xas_split_alloc() needs to be re-organized.

So the simplest fix is probably to just add a

                        if (xas_error(&xas))
                                goto error;
                }
+               xas_reset(&xas);
                xas_lock_irq(&xas);
                xas_for_each_conflict(&xas, entry) {
                        old = entry;

in __filemap_add_folio() in mm/filemap.c

(The above is obviously a whitespace-damaged pseudo-patch for the
pre-6758c1128ceb state. I don't actually carry a stable tree around on
my laptop, but I hope it's clear enough what I'm rambling about)

               Linus