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: <30a9baa7-5368-4ef7-9091-80d170cfb5c6@suse.cz>
Date: Thu, 6 Nov 2025 12:12:10 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 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>