linux-kernel - Re: [PATCH V1 1/2] docs/mm: explain when and why rmap locks need to be taken during mremap()

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Message-ID: <87v7mabl9g.fsf@trenco.lwn.net>
Date: Tue, 26 Aug 2025 01:22:03 -0600
From: Jonathan Corbet <corbet@....net>
To: Harry Yoo <harry.yoo@...cle.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Suren Baghdasaryan <surenb@...gle.com>, "Liam
 R . Howlett" <Liam.Howlett@...cle.com>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, David Hildenbrand <david@...hat.com>, Kees
 Cook <kees@...nel.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Shakeel Butt <shakeel.butt@...ux.dev>,
 Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>, Jann Horn
 <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>, Rik van Riel
 <riel@...riel.com>, linux-mm@...ck.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Harry Yoo <harry.yoo@...cle.com>
Subject: Re: [PATCH V1 1/2] docs/mm: explain when and why rmap locks need to
 be taken during mremap()

Harry Yoo <harry.yoo@...cle.com> writes:

> While move_ptes() has a comment explaining why rmap locks are needed,
> Documentation/mm/process_addrs.rst does not. Without being aware of that
> comment, I spent hours figuring out how things could go wrong and why,
> in some cases, rmap locks can be safely skipped.
>
> Add a more comprehensive explanation to the documentation to save time
> for others.
>
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  Documentation/mm/process_addrs.rst | 32 ++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
> index be49e2a269e4..ee7c0dba339e 100644
> --- a/Documentation/mm/process_addrs.rst
> +++ b/Documentation/mm/process_addrs.rst
> @@ -744,6 +744,38 @@ You can observe this in the :c:func:`!mremap` implementation in the functions
>  :c:func:`!take_rmap_locks` and :c:func:`!drop_rmap_locks` which perform the rmap
>  side of lock acquisition, invoked ultimately by :c:func:`!move_page_tables`.
>  
> +.. note:: If :c:func:`!mremap()` -> :c:func:`!move_ptes()` does not take rmap
> +          locks, :c:func:`!rmap_walk()` may miss a pte for the folio.
> +
> +          The problematic sequence is as follows:

Please don't use :c:func: - just write function() and all the right
things will happen.  (For extra credit, fix the existing usages :)

Thanks,

jon