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

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Message-ID: <043dcbdb-e069-46e7-8f79-8fdaf354fb44@lucifer.local>
Date: Thu, 6 Nov 2025 14:54:33 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Alice Ryhl <aliceryhl@...gle.com>
Subject: Re: [PATCH v2 1/5] mm: introduce VM_MAYBE_GUARD and make visible in
 /proc/$pid/smaps

+cc Alice for rust stuff

On Thu, Nov 06, 2025 at 02:27:56PM +0000, Pedro Falcato wrote:
> 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@...cle.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:     =============================================================

Sigh :) OK will fix.

>
>
> >
> >  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").

That's a bit unhelpful if rust can't cope with extremely basic assignments like
that and we just have to know to add helpers :/

We do BIT() stuff for e.g. VM_HIGH_ARCH_n, VM_UFFD_MINOR_BIT,
VM_ALLOW_ANY_UNCACHED_BIT, VM_DROPPABLE_BIT and VM_SEALED_BIT too and no such
helpers there, So not sure if this is required?

Alice - why is it these 'non-trivial' defines were fine but VM_MERGEABLE was
problematic? That seems strange.

I see [0], so let me build rust here and see if it moans, if it moans I'll add
it.

[0]:https://lore.kernel.org/oe-kbuild-all/CANiq72kOhRdGtQe2UVYmDLdbw6VNkiMtdFzkQizsfQV0gLY1Hg@mail.gmail.com/

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

Thanks

>
> --
> Pedro