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

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Nov 06, 2025 at 02:54:33PM +0000, Lorenzo Stoakes wrote:
> +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@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:     =============================================================
> 
> 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/

When you use #define to declare a constant whose right-hand-side
contains a function-like macro such as BIT(), bindgen does not define a
Rust version of that constant. However, VM_MAYBE_GUARD is not referenced
in Rust anywhere, so that isn't a problem.

It was a problem with VM_MERGEABLE because rust/kernel/mm/virt.rs
references it.

Note that it's only the combination of #define and function-like macro
that triggers this condition. If the constant is defined using another
mechanism such as enum {}, then bindgen will generate the constant no
matter how complex the right-hand-side is. The problem is that bindgen
can't tell whether a #define is just a constant or not.

Alice