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 <[email protected]>
> > Reviewed-by: Vlastimil Babka <[email protected]>
> > Signed-off-by: Lorenzo Stoakes <[email protected]>
> > ---
> > 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) <[email protected]>
Thanks!
>
> --
> Cheers
>
> David
Cheers, Lorenzo
