linux-kernel - Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd

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Message-ID: <Z7e7iYNvGweeGsRU@x1.local>
Date: Thu, 20 Feb 2025 18:32:25 -0500
From: Peter Xu <peterx@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: david@...hat.com, Liam.Howlett@...cle.com, aarcange@...hat.com,
	akpm@...ux-foundation.org, axelrasmussen@...gle.com,
	bgeffon@...gle.com, brauner@...nel.org, hughd@...gle.com,
	jannh@...gle.com, kaleshsingh@...gle.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	lokeshgidra@...gle.com, mhocko@...e.com, ngeoffray@...gle.com,
	rppt@...nel.org, ryan.roberts@....com, shuah@...nel.org,
	surenb@...gle.com, v-songbaohua@...o.com, viro@...iv.linux.org.uk,
	willy@...radead.org, zhangpeng362@...wei.com,
	zhengtangquan@...o.com, yuzhao@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters
 swapcache

On Thu, Feb 20, 2025 at 10:21:01PM +1300, Barry Song wrote:
> 2. src_anon_vma and its lock – swapcache doesn’t require it（folio is not mapped）

Could you help explain what guarantees the rmap walk not happen on a
swapcache page?

I'm not familiar with this path, though at least I see damon can start a
rmap walk on PageAnon almost with no locking..  some explanations would be
appreciated.

-- 
Peter Xu