linux-kernel - Re: [PATCH] mm: swap: prevent possible data-race in __try_to_reclaim

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Message-ID: <Zv_87TBZnh2lIwyH@casper.infradead.org>
Date: Fri, 4 Oct 2024 15:34:21 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jeongjun Park <aha310510@...il.com>
Cc: akpm@...ux-foundation.org, kasong@...cent.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	syzbot+fa43f1b63e3aa6f66329@...kaller.appspotmail.com
Subject: Re: [PATCH] mm: swap: prevent possible data-race in
 __try_to_reclaim_swap

On Fri, Oct 04, 2024 at 11:25:04PM +0900, Jeongjun Park wrote:
> A report [1] was uploaded from syzbot.
> 
> In the previous commit 862590ac3708 ("mm: swap: allow cache reclaim to skip 
> slot cache"), the __try_to_reclaim_swap() function reads offset and nr_pages 
> from folio without folio_lock protection. 

Umm.  You don't need folio_lock to read nr_pages.  Holding a refcount
is sufficient to stabilise nr_pages.  I cannot speak to folio->swap
though (and the KCSAN report does appear to be pointing to folio->swap).