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 <[email protected]> > > Cc: Andrew Morton <[email protected]> > > Cc: Simona Vetter <[email protected]> > > Cc: Dave Airlie <[email protected]> > > Cc: Alistair Popple <[email protected]> > > Cc: <[email protected]> > > Cc: <[email protected]> > > Cc: <[email protected]> > > > > Assisted-by: GitHub Copilot:claude-sonnet-4.6 # Documentation only. > > Signed-off-by: Thomas Hellström <[email protected]> > > --- > > 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
