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

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Message-ID: <xnsn5rfqigbm5ryjtbf2rtfotneiwygzesvyfdxiqrzlyzljdr@tmbht4ggnjcv>
Date: Thu, 30 Oct 2025 16:16:20 +0000
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	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>
Subject: Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for
 guard regions

On Wed, Oct 29, 2025 at 04:50:31PM +0000, Lorenzo Stoakes wrote:
> Currently, if a user needs to determine if guard regions are present in a
> range, they have to scan all VMAs (or have knowledge of which ones might
> have guard regions).
> 
> Since commit 8e2f2aeb8b48 ("fs/proc/task_mmu: add guard region bit to
> pagemap") and the related commit a516403787e0 ("fs/proc: extend the
> PAGEMAP_SCAN ioctl to report guard regions"), users can use either
> /proc/$pid/pagemap or the PAGEMAP_SCAN functionality to perform this
> operation at a virtual address level.
> 
> This is not ideal, and it gives no visibility at a /proc/$pid/smaps level
> that guard regions exist in ranges.
> 
> This patch remedies the situation by establishing a new VMA flag,
> VM_MAYBE_GUARD, to indicate that a VMA may contain guard regions (it is
> uncertain because we cannot reasonably determine whether a
> MADV_GUARD_REMOVE call has removed all of the guard regions in a VMA, and
> additionally VMAs may change across merge/split).
> 
> We utilise 0x800 for this flag which makes it available to 32-bit
> architectures also, a flag that was previously used by VM_DENYWRITE, which
> was removed in commit 8d0920bde5eb ("mm: remove VM_DENYWRITE") and hasn't
> bee reused yet.
> 
> The MADV_GUARD_INSTALL madvise() operation now must take an mmap write
> lock (and also VMA write lock) whereas previously it did not, but this
> seems a reasonable overhead.

Do you though? Could it be possible to simply atomically set the flag with
the read lock held? This would make it so we can't split the VMA (and tightly
define what "may have a guard page"), but it sounds much better than introducing
lock contention. I don't think it is reasonable to add a write lock to a feature
that may be used by such things as thread stack allocation, malloc, etc.

-- 
Pedro