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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 Subject: Re: [PATCH v3 1/8] mm: introduce VM_MAYBE_GUARD and make visible in /proc/$pid/smaps Message-ID: <057b6f29-2655-440f-9db5-9340012a7f35@lucifer.local> References: <94d1e9c6c6dd8a4de1f2a8022ca92e2e320730ff.1762531708.git.lorenzo.stoakes@oracle.com> <9e617d7b-afd7-465b-b075-32b02257b90b@kernel.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9e617d7b-afd7-465b-b075-32b02257b90b@kernel.org> X-ClientProxiedBy: LO6P123CA0017.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:313::7) To 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04:11:47PM +0100, David Hildenbrand (Red Hat) wrote: > On 07.11.25 17:11, Lorenzo Stoakes wrote: > > Currently, if a user needs to determine if guard regions are present in a > > range, they have to scan all VMAs (or have knowledge of which ones might > > have guard regions). > > > > Since commit 8e2f2aeb8b48 ("fs/proc/task_mmu: add guard region bit to > > pagemap") and the related commit a516403787e0 ("fs/proc: extend the > > PAGEMAP_SCAN ioctl to report guard regions"), users can use either > > /proc/$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 > > 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. 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) Thanks! > > -- > Cheers > > David Cheers, Lorenzo