linux-kernel - Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for guard regions

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Message-ID: <15d3b854-a2f2-42a3-b2ab-3a8c822dae9f@lucifer.local>
Date: Mon, 3 Nov 2025 09:34:31 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Pedro Falcato <pfalcato@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Barry Song <21cnbao@...il.com>
Subject: Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for
 guard regions

On Fri, Oct 31, 2025 at 04:12:51PM -0700, Suren Baghdasaryan wrote:
>
> Overall, I think it should be possible to set this flag atomically
> under VMA read-lock. However, if you introduce new vm_flags
> manipulation functions, please make sure they can't be used for other
> vm_flags. In Android I've seen several "interesting" attempts to
> update vm_flags under a read-lock (specifically in the page-fault
> path) and had to explain why that's a bad idea.

Yeah agreed, so the idea would be to absolutely ring-fence any flag we do
this with entirely. Probably a VM_WARN_ON_ONCE() for anybody trying it with
other flags so bots can catch anybody being naughty.

That kind of 'interesting' stuff is another reason I prefer to really limit
what drivers can do btw ;)

Will have a respin with this idea applied relatively soon hopefully.

Cheers, Lorenzo