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

["David Hildenbrand (Arm)" <david@kernel.org>]

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.

-- 
Cheers,

David