Re: [PATCH v3 1/4] mm/mmu_notifier: Allow two-pass struct mmu_interval_notifiers

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2026-03-04 at 20:45 +0100, David Hildenbrand (Arm) wrote:
> On 3/3/26 14:34, Thomas Hellström wrote:
> > GPU use-cases for mmu_interval_notifiers with hmm often involve
> > starting a gpu operation and then waiting for it to complete.
> > These operations are typically context preemption or TLB flushing.
> > 
> > With single-pass notifiers per GPU this doesn't scale in
> > multi-gpu scenarios. In those scenarios we'd want to first start
> > preemption- or TLB flushing on all GPUs and as a second pass wait
> > for them to complete.
> > 
> > One can do this on per-driver basis multiplexing per-driver
> > notifiers but that would mean sharing the notifier "user" lock
> > across all GPUs and that doesn't scale well either, so adding
> > support
> > for multi-pass in the core appears to be the right choice.
> > 
> > Implement two-pass capability in the mmu_interval_notifier. Use a
> > linked list for the final passes to minimize the impact for
> > use-cases that don't need the multi-pass functionality by avoiding
> > a second interval tree walk, and to be able to easily pass data
> > between the two passes.
> > 
> > v1:
> > - Restrict to two passes (Jason Gunthorpe)
> > - Improve on documentation (Jason Gunthorpe)
> > - Improve on function naming (Alistair Popple)
> > v2:
> > - Include the invalidate_finish() callback in the
> >   struct mmu_interval_notifier_ops.
> > - Update documentation (GitHub Copilot:claude-sonnet-4.6)
> > - Use lockless list for list management.
> > v3:
> > - Update kerneldoc for the struct
> > mmu_interval_notifier_finish::list member
> >   (Matthew Brost)
> > - Add a WARN_ON_ONCE() checking for NULL invalidate_finish() op if
> >   if invalidate_start() is non-NULL. (Matthew Brost)
> > 
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Simona Vetter <simona.vetter@ffwll.ch>
> > Cc: Dave Airlie <airlied@gmail.com>
> > Cc: Alistair Popple <apopple@nvidia.com>
> > Cc: <dri-devel@lists.freedesktop.org>
> > Cc: <linux-mm@kvack.org>
> > Cc: <linux-kernel@vger.kernel.org>
> > 
> > Assisted-by: GitHub Copilot:claude-sonnet-4.6 # Documentation only.
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > ---
> >  include/linux/mmu_notifier.h | 38 +++++++++++++++++++++
> >  mm/mmu_notifier.c            | 65 +++++++++++++++++++++++++++++++-
> > ----
> >  2 files changed, 94 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/linux/mmu_notifier.h
> > b/include/linux/mmu_notifier.h
> > index 07a2bbaf86e9..37b683163235 100644
> > --- a/include/linux/mmu_notifier.h
> > +++ b/include/linux/mmu_notifier.h
> > @@ -233,16 +233,54 @@ struct mmu_notifier {
> >  	unsigned int users;
> >  };
> >  
> > +/**
> > + * struct mmu_interval_notifier_finish - mmu_interval_notifier
> > two-pass abstraction
> > + * @link: Lockless list link for the notifiers pending pass list
> > + * @notifier: The mmu_interval_notifier for which the finish pass
> > is called.
> > + *
> > + * Allocate, typically using GFP_NOWAIT in the interval notifier's
> > first pass.
> 
> Might want to make it clear that the fist pass is "start" and the
> second
> pass is "finish".
> 
> Two-pass makes it sound like we'd be calling the same operation
> (e.g.,
> invalidate() ) twice.
> 
> > + * If allocation fails (which is not unlikely under memory
> > pressure), fall back
> > + * to single-pass operation. 
> 
> Do you mean that the core will fallback (calling invalidate() ) or
> that
> it's the responsibility of the notifier to behave as if invalidate()
> would be called to then return finish=NULL? I assume the latter.
> 
> Maybe this should be documented for @invalidate_start instead.
> (behave
> like invalidate() if @finish is %NULL on return etc)
> 
> > Note that with a large number of notifiers
> > + * implementing two passes, allocation with GFP_NOWAIT will become
> > increasingly
> > + * likely to fail, so consider implementing a small pool instead
> > of using
> > + * kmalloc() allocations.
> > + *
> > + * If the implementation needs to pass data between the two
> > passes,
> > + * the recommended way is to embed struct
> > mmu_interval_notifier_finish into a larger
> > + * structure that also contains the data needed to be shared. Keep
> > in mind that
> > + * a notifier callback can be invoked in parallel, and each
> > invocation needs its
> > + * own struct mmu_interval_notifier_finish.
> > + */
> > +struct mmu_interval_notifier_finish {
> > +	struct llist_node link;
> > +	struct mmu_interval_notifier *notifier;
> > +};
> > +
> >  /**
> >   * struct mmu_interval_notifier_ops
> >   * @invalidate: Upon return the caller must stop using any SPTEs
> > within this
> >   *              range. This function can sleep. Return false only
> > if sleeping
> >   *              was required but
> > mmu_notifier_range_blockable(range) is false.
> > + * @invalidate_start: Similar to @invalidate, but intended for
> > two-pass notifier
> > + *                    callbacks where the call to
> > @invalidate_start is the first
> > + *                    pass and any struct
> > mmu_interval_notifier_finish pointer
> > + *                    returned in the @finish parameter describes
> > the final pass.
> > + *                    If @finish is %NULL on return, then no final
> > pass will be
> > + *                    called.
> 
> Is @finish guaranteed to be set to %NULL before the call? The
> existing
> code does it, but is it something notifiers can rely on?
> 
> > + * @invalidate_finish: Called as the second pass for any notifier
> > that returned
> > + *                     a non-NULL @finish from @invalidate_start.
> > The @finish
> > + *                     pointer passed here is the same one
> > returned by
> > + *                     @invalidate_start.
> >   */
> >  struct mmu_interval_notifier_ops {
> >  	bool (*invalidate)(struct mmu_interval_notifier
> > *interval_sub,
> >  			   const struct mmu_notifier_range *range,
> >  			   unsigned long cur_seq);
> > +	bool (*invalidate_start)(struct mmu_interval_notifier
> > *interval_sub,
> > +				 const struct mmu_notifier_range
> > *range,
> > +				 unsigned long cur_seq,
> > +				 struct
> > mmu_interval_notifier_finish **finish);
> > +	void (*invalidate_finish)(struct
> > mmu_interval_notifier_finish *finish);
> >  };
> 
> 
> Nothing else jumped at me, and the idea makes sense.


Thanks. I sent out a v4 addressing the above and to a wider audience.

Thanks,
Thomas