From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8F6CD27F724; Mon, 17 Nov 2025 15:11:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763392316; cv=none; b=MTdIJ7RQnfbeU5ojGYRleb1EpGH1fVob/ELPyomd7WejQ9KhesIvzGwuPAqOiEK3TBU0WStQ5BPZTL49njaTRWIUNmT5prTCcpv9yHVIcawmg5caT5Rfyg7HXI3nmX5UG8C3ctJgbPkL3ssHDXnVmPnaj40H8ItJozE5QwpupYg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763392316; c=relaxed/simple; bh=TwfGBepi2IUAU/4JFAvHJYTM1sRw3y0VTMA2H2PFuPQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=RoFe4gctnLOUMMyB4Jimv0yv8Un5NaEsFMlEsgJlL92PqFQmVsrPLSzj8SUNm23p4fezKlr+SkBgRHtm/nje6HQvCzcSu/EMPH8Eps00CLHW0o9+KqdzmQOG78S0lc6eLAsWqbUYYnd8/LVqDgqg5EBLWy1w9fboqXOimsoo174= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fYFQfp98; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fYFQfp98" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9D213C113D0; Mon, 17 Nov 2025 15:11:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763392316; bh=TwfGBepi2IUAU/4JFAvHJYTM1sRw3y0VTMA2H2PFuPQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=fYFQfp98q8CZPq2D3/1HwhNKfQ02GOU/p/lXsiGElTjPUzEmC11RUcdhfE0uh+LZe hkTadqxNsT86x26pV78yE9cHtXOZohKJc0CnrhNLYiIZQceAFHoqFiPE78DidXhXLN tFAKq7VHPXZt3xIHdv8rlwuvCmnXJCirwhXCPCvCDSY+GaHO9QVUWndBeMs4LtVKhB vXQzJeyLQ5i7XRjJafFG+RpCQs24WSu+XrfAhKl64rdC/PRf63+PQQaFX6cPXenQKt k5jwAq/pMGJUEEMPjo9TEaBxkVgMEBH/vcGjv4clAdluHxZE2X66gZlaSyNyJg/OV1 zEIIjPZilJmXA== Message-ID: <9e617d7b-afd7-465b-b075-32b02257b90b@kernel.org> Date: Mon, 17 Nov 2025 16:11:47 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/8] mm: introduce VM_MAYBE_GUARD and make visible in /proc/$pid/smaps To: Lorenzo Stoakes , Andrew Morton Cc: Jonathan Corbet , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jann Horn , Pedro Falcato , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Andrei Vagin References: <94d1e9c6c6dd8a4de1f2a8022ca92e2e320730ff.1762531708.git.lorenzo.stoakes@oracle.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <94d1e9c6c6dd8a4de1f2a8022ca92e2e320730ff.1762531708.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.11.25 17:11, Lorenzo Stoakes wrote: > Currently, if a user needs to determine if guard regions are present in a > range, they have to scan all VMAs (or have knowledge of which ones might > have guard regions). > > Since commit 8e2f2aeb8b48 ("fs/proc/task_mmu: add guard region bit to > pagemap") and the related commit a516403787e0 ("fs/proc: extend the > PAGEMAP_SCAN ioctl to report guard regions"), users can use either > /proc/$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 > Reviewed-by: Vlastimil Babka > Signed-off-by: Lorenzo Stoakes > --- > 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. Acked-by: David Hildenbrand (Red Hat) -- Cheers David