Re: [PATCH v2 1/5] mm: introduce VM_MAYBE_GUARD and make visible in /proc/$pid/smaps

[Pedro Falcato <pfalcato@suse.de>]

On Thu, Nov 06, 2025 at 10:46:12AM +0000, 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@oracle.com>
> ---
>  Documentation/filesystems/proc.rst | 1 +
>  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   | 3 +++
>  6 files changed, 13 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 0b86a8022fa1..b8a423ca590a 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -591,6 +591,7 @@ 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)
>      ==    =======================================

The nittiest
of nits:     =============================================================


>  
>  Note that there is no guarantee that every flag and associated mnemonic will
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 8a9894aefbca..a420dcf9ffbb 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1147,6 +1147,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>  		[ilog2(VM_MAYSHARE)]	= "ms",
>  		[ilog2(VM_GROWSDOWN)]	= "gd",
>  		[ilog2(VM_PFNMAP)]	= "pf",
> +		[ilog2(VM_MAYBE_GUARD)]	= "gu",
>  		[ilog2(VM_LOCKED)]	= "lo",
>  		[ilog2(VM_IO)]		= "io",
>  		[ilog2(VM_SEQ_READ)]	= "sr",
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6e5ca5287e21..2a5516bff75a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -271,6 +271,8 @@ extern struct rw_semaphore nommu_region_sem;
>  extern unsigned int kobjsize(const void *objp);
>  #endif
>  
> +#define VM_MAYBE_GUARD_BIT 11
> +
>  /*
>   * vm_flags in vm_area_struct, see mm_types.h.
>   * When changing, update also include/trace/events/mmflags.h
> @@ -296,6 +298,7 @@ extern unsigned int kobjsize(const void *objp);
>  #define VM_UFFD_MISSING	0
>  #endif /* CONFIG_MMU */
>  #define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct page", just pure PFN */
> +#define VM_MAYBE_GUARD	BIT(VM_MAYBE_GUARD_BIT)	/* The VMA maybe contains guard regions. */

Don't we also need an adjustment on the rust side for this BIT()? Like we
for f04aad36a07c ("mm/ksm: fix flag-dropping behavior in ksm_madvise").

In any case:
Reviewed-by: Pedro Falcato <pfalcato@suse.de>

-- 
Pedro