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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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@suse.de>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.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.

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

-- 
Cheers

David