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: <ZurXAco1BKqf8I2E@casper.infradead.org>
Date: Wed, 18 Sep 2024 14:34:57 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Chris Mason <clm@...a.com>
Cc: Jens Axboe <axboe@...nel.dk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.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 Wed, Sep 18, 2024 at 11:28:52AM +0200, Chris Mason wrote:
> I think the bug was in __filemap_add_folio()'s usage of xarray_split_alloc()
> and the tree changing before taking the lock.  It's just a guess, but that
> was always my biggest suspect.

Oh god, that's it.

there should have been an xas_reset() after calling xas_split_alloc().

and 6758c1128ceb calls xas_reset() after calling xas_split_alloc().

i wonder if xas_split_alloc() should call xas_reset() to prevent this
from ever being a problem again?