Message-ID: <69c73e1e-c520-42e7-af0a-65f17f5e081e@lucifer.local>
Date: Thu, 6 Nov 2025 15:03:09 +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>
Subject: Re: [PATCH v2 2/5] mm: add atomic VMA flags, use VM_MAYBE_GUARD as
 such

On Thu, Nov 06, 2025 at 02:45:06PM +0000, Pedro Falcato wrote:
> On Thu, Nov 06, 2025 at 10:46:13AM +0000, Lorenzo Stoakes wrote:
> > This patch adds the ability to atomically set VMA flags with only the mmap
> > read/VMA read lock held.
> >
> > As this could be hugely problematic for VMA flags in general given that all
> > other accesses are non-atomic and serialised by the mmap/VMA locks, we
> > implement this with a strict allow-list - that is, only designated flags
> > are allowed to do this.
> >
> > We make VM_MAYBE_GUARD one of these flags, and then set it under the mmap
> > read flag upon guard region installation.
> >
> > The places where this flag is used currently and matter are:
> >
> > * VMA merge - performed under mmap/VMA write lock, therefore excluding
> >   racing writes.
> >
> > * /proc/$pid/smaps - can race the write, however this isn't meaningful as
> >   the flag write is performed at the point of the guard region being
> >   established, and thus an smaps reader can't reasonably expect to avoid
> >   races. Due to atomicity, a reader will observe either the flag being set
> >   or not. Therefore consistency will be maintained.
> >
> > In all other cases the flag being set is irrelevant and atomicity
> > guarantees other flags will be read correctly.
>
> Probably important to write down that the only reason why this doesn't make
> KCSAN have a small stroke is that we are only changing one bit. i.e we can
> only have one bit of atomic flags before annotating every reader.
>
> (Source: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/kcsan/permissive.h#n51)

That seems a bit specific and technical though? I guess since Vlasta is asking
for maximum commit message pedantry here the more the merrier...

>
> > We additionally update madvise_guard_install() to ensure that
> > anon_vma_prepare() is set for anonymous VMAs to maintain consistency with
> > the assumption that any anonymous VMA with page tables will have an
> > anon_vma set, and any with an anon_vma unset will not have page tables
> > established.
>
> Isn't that what we already had? Or do you mean "*only* set for anonymous VMAs"?

Yes... I'm going to expand on this explanation as per Vlasta to make it
extremely clear anyway.

>
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> With the nits below and above addressed:
> Reviewed-by: Pedro Falcato <pfalcato@...e.de>

Thanks, though I disagree with nit below.

>
> > ---
> >  include/linux/mm.h | 23 +++++++++++++++++++++++
> >  mm/madvise.c       | 22 ++++++++++++++--------
> >  2 files changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 2a5516bff75a..2ea65c646212 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -518,6 +518,9 @@ extern unsigned int kobjsize(const void *objp);
> >  /* This mask represents all the VMA flag bits used by mlock */
> >  #define VM_LOCKED_MASK	(VM_LOCKED | VM_LOCKONFAULT)
> >
> > +/* These flags can be updated atomically via VMA/mmap read lock. */
> > +#define VM_ATOMIC_SET_ALLOWED VM_MAYBE_GUARD
> > +
> >  /* Arch-specific flags to clear when updating VM flags on protection change */
> >  #ifndef VM_ARCH_CLEAR
> >  # define VM_ARCH_CLEAR	VM_NONE
> > @@ -860,6 +863,26 @@ static inline void vm_flags_mod(struct vm_area_struct *vma,
> >  	__vm_flags_mod(vma, set, clear);
> >  }
> >
> > +/*
> > + * Set VMA flag atomically. Requires only VMA/mmap read lock. Only specific
> > + * valid flags are allowed to do this.
> > + */
> > +static inline void vma_flag_set_atomic(struct vm_area_struct *vma,
> > +				       int bit)
> > +{
> > +	const vm_flags_t mask = BIT(bit);
> > +
> > +	/* mmap read lock/VMA read lock must be held. */
> > +	if (!rwsem_is_locked(&vma->vm_mm->mmap_lock))
> > +		vma_assert_locked(vma);
> > +
> > +	/* Only specific flags are permitted */
> > +	if (WARN_ON_ONCE(!(mask & VM_ATOMIC_SET_ALLOWED)))
> > +		return;
>
> VM_WARN_ON_ONCE?

No, this was on puurpose - I don't want drivers (incl. out of tree) abusing this
so I think this should be runtime and explicitly clear. See Suren's comment on
last revision of series.

Obviously we should never be giving drivers naked vma pointers where this
matters (and actually not sure exactly where it would), my mmap_prepare series
is working to mitigate this though it's in a situation where the locking doesn't
matter.

Also you can't use VM_WARN_ON_ONCE() that way, for some reason we don't have it
return a value, go figure.

>
> --
> Pedro

Thanks, Lorenzo

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