linux-kernel - Re: [PATCH v2 1/5] mm: introduce VM_MAYBE_GUARD and make visible in /proc/$pid/smaps

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Message-ID: <32e6be58-1738-4f1f-9ce1-78209d4e5174@lucifer.local>
Date: Thu, 6 Nov 2025 13:56:49 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: 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>,
        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>, Pedro Falcato <pfalcato@...e.de>,
        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 v2 1/5] mm: introduce VM_MAYBE_GUARD and make visible in
 /proc/$pid/smaps

On Thu, Nov 06, 2025 at 12:12:10PM +0100, Vlastimil Babka wrote:
> On 11/6/25 11:46, 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.
> >
> > We also update the smaps logic and documentation to identify these VMAs.
> >
> > Another major use of this functionality is that we can use it to identify
> > that we ought to copy page tables on fork.
> >
> > We do not actually implement usage of this flag in mm/madvise.c yet as we
> > need to allow some VMA flags to be applied atomically under mmap/VMA read
> > lock in order to avoid the need to acquire a write lock for this purpose.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>

Thanks