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: <a32e718c-e3fc-4f9f-9cb5-3df539664743@lucifer.local>
Date: Thu, 6 Nov 2025 14:58:24 +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

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

I built with CONFIG_RUST=y and everything compiles ok so seems rust is fine with
it?

Strange that we need it for some things but not others though?