linux-kernel - Re: [PATCH v3 1/8] mm: introduce VM_MAYBE_GUARD and make visible in /proc/$pid/smaps

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Message-ID: <057b6f29-2655-440f-9db5-9340012a7f35@lucifer.local>
Date: Tue, 18 Nov 2025 07:36:02 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        "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>, Pedro Falcato <pfalcato@...e.de>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lance Yang <lance.yang@...ux.dev>, 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 v3 1/8] mm: introduce VM_MAYBE_GUARD and make visible in
 /proc/$pid/smaps

On Mon, Nov 17, 2025 at 04:11:47PM +0100, David Hildenbrand (Red Hat) wrote:
> On 07.11.25 17:11, 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.
> >
> > Reviewed-by: Pedro Falcato <pfalcato@...e.de>
> > Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> >   Documentation/filesystems/proc.rst | 5 +++--
> >   fs/proc/task_mmu.c                 | 1 +
> >   include/linux/mm.h                 | 3 +++
> >   include/trace/events/mmflags.h     | 1 +
> >   mm/memory.c                        | 4 ++++
> >   tools/testing/vma/vma_internal.h   | 1 +
> >   6 files changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > index 0b86a8022fa1..8256e857e2d7 100644
> > --- a/Documentation/filesystems/proc.rst
> > +++ b/Documentation/filesystems/proc.rst
> > @@ -553,7 +553,7 @@ otherwise.
> >   kernel flags associated with the particular virtual memory area in two letter
> >   encoded manner. The codes are the following:
> > -    ==    =======================================
> > +    ==    =============================================================
> >       rd    readable
> >       wr    writeable
> >       ex    executable
> > @@ -591,7 +591,8 @@ encoded manner. The codes are the following:
> >       sl    sealed
> >       lf    lock on fault pages
> >       dp    always lazily freeable mapping
> > -    ==    =======================================
> > +    gu    maybe contains guard regions (if not set, definitely doesn't)
> > +    ==    =============================================================
>
>
> In general LGTM, BUT in the context of this patch where the flag is never
> set, that's not entirely correct ;) It made sense after staring at patch #5.

Yeah I realise that's a bit of compromise but I think relatively benign
in order to split up the series sensibly :)

>
> Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>

Thanks!

>
> --
> Cheers
>
> David

Cheers, Lorenzo