Re: [PATCH RESEND v3 10/10] mm/vma: add and use vma_assert_stabilised()

Lorenzo Stoakes
2026-01-23T18:54:59Z

On Thu, Jan 22, 2026 at 02:12:25PM -0800, Suren Baghdasaryan wrote:
> On Thu, Jan 22, 2026 at 5:02 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > Sometimes we wish to assert that a VMA is stable, that is - the VMA cannot
> > be changed underneath us. This will be the case if EITHER the VMA lock or
> > the mmap lock is held.
> >
> > In order to do so, we introduce a new assert vma_assert_stablised() - this
>
> s/vma_assert_stablised/vma_assert_stabilised

Oops will fix.

>
> > will make a lockdep assert if lockdep is enabled AND the VMA is
> > read-locked.
> >
> > Currently lockdep tracking for VMA write locks is not implemented, so it
> > suffices to check in this case that we have either an mmap read or write
> > semaphore held.
> >
> > Note that because the VMA lock uses the non-standard vmlock_dep_map naming
> > convention, we cannot use lockdep_assert_is_write_held() so have to open
> > code this ourselves via lockdep-asserting that
> > lock_is_held_type(&vma->vmlock_dep_map, 0).
> >
> > We have to be careful here - for instance when merging a VMA, we use the
> > mmap write lock to stabilise the examination of adjacent VMAs which might
> > be simultaneously VMA read-locked whilst being faulted in.
> >
> > If we were to assert VMA read lock using lockdep we would encounter an
> > incorrect lockdep assert.
> >
> > Also, we have to be careful about asserting mmap locks are held - if we try
> > to address the above issue by first checking whether mmap lock is held and
> > if so asserting it via lockdep, we may find that we were raced by another
> > thread acquiring an mmap read lock simultaneously that either we don't
> > own (and thus can be released any time - so we are not stable) or was
> > indeed released since we last checked.
> >
> > So to deal with these complexities we end up with either a precise (if
> > lockdep is enabled) or imprecise (if not) approach - in the first instance
> > we assert the lock is held using lockdep and thus whether we own it.
> >
> > If we do own it, then the check is complete, otherwise we must check for
> > the VMA read lock being held (VMA write lock implies mmap write lock so the
> > mmap lock suffices for this).
> >
> > If lockdep is not enabled we simply check if the mmap lock is held and risk
> > a false negative (i.e. not asserting when we should do).
> >
> > There are a couple places in the kernel where we already do this
> > stabliisation check - the anon_vma_name() helper in mm/madvise.c and
> > vma_flag_set_atomic() in include/linux/mm.h, which we update to use
> > vma_assert_stabilised().
> >
> > This change abstracts these into vma_assert_stabilised(), uses lockdep if
> > possible, and avoids a duplicate check of whether the mmap lock is held.
> >
> > This is also self-documenting and lays the foundations for further VMA
> > stability checks in the code.
>
> So, is the lockdep addition the only functional change here?

Yurp. Wil add to commit msg.

>
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Thanks :)

And also thanks to you for the review in general! :)