Re: [PATCH v2 1/4] mm/mmu_notifier: Allow two-pass struct mmu_interval_notifiers

[Matthew Brost]

On Mon, Mar 02, 2026 at 05:32:45PM +0100, 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.
> 
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>

I thought Jason had given a RB on previous revs - did you drop because
enough has changed?
 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Simona Vetter <simona.vetter@xxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: <dri-devel@xxxxxxxxxxxxxxxxxxxxx>
> Cc: <linux-mm@xxxxxxxxx>
> Cc: <linux-kernel@xxxxxxxxxxxxxxx>
> 
> Assisted-by: GitHub Copilot:claude-sonnet-4.6 # Documentation only.
> Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> ---
>  include/linux/mmu_notifier.h | 38 +++++++++++++++++++++
>  mm/mmu_notifier.c            | 64 +++++++++++++++++++++++++++++++-----
>  2 files changed, 93 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index 07a2bbaf86e9..de0e742ea808 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: List link for the notifiers pending pass list

Lockless 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.
> + * If allocation fails (which is not unlikely under memory pressure), fall back
> + * to single-pass operation. 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.
> + * @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);

Should we complain somewhere if a caller registers a notifier with
invalidate_start set but not invalidate_finish?

Matt

>  };
>  
>  struct mmu_interval_notifier {
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index a6cdf3674bdc..38acd5ef8eb0 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -260,6 +260,15 @@ mmu_interval_read_begin(struct mmu_interval_notifier *interval_sub)
>  }
>  EXPORT_SYMBOL_GPL(mmu_interval_read_begin);
>  
> +static void mn_itree_finish_pass(struct llist_head *finish_passes)
> +{
> +	struct llist_node *first = llist_reverse_order(__llist_del_all(finish_passes));
> +	struct mmu_interval_notifier_finish *f, *next;
> +
> +	llist_for_each_entry_safe(f, next, first, link)
> +		f->notifier->ops->invalidate_finish(f);
> +}
> +
>  static void mn_itree_release(struct mmu_notifier_subscriptions *subscriptions,
>  			     struct mm_struct *mm)
>  {
> @@ -271,6 +280,7 @@ static void mn_itree_release(struct mmu_notifier_subscriptions *subscriptions,
>  		.end = ULONG_MAX,
>  	};
>  	struct mmu_interval_notifier *interval_sub;
> +	LLIST_HEAD(finish_passes);
>  	unsigned long cur_seq;
>  	bool ret;
>  
> @@ -278,11 +288,27 @@ static void mn_itree_release(struct mmu_notifier_subscriptions *subscriptions,
>  		     mn_itree_inv_start_range(subscriptions, &range, &cur_seq);
>  	     interval_sub;
>  	     interval_sub = mn_itree_inv_next(interval_sub, &range)) {
> -		ret = interval_sub->ops->invalidate(interval_sub, &range,
> -						    cur_seq);
> +		if (interval_sub->ops->invalidate_start) {
> +			struct mmu_interval_notifier_finish *finish = NULL;
> +
> +			ret = interval_sub->ops->invalidate_start(interval_sub,
> +								  &range,
> +								  cur_seq,
> +								  &finish);
> +			if (ret && finish) {
> +				finish->notifier = interval_sub;
> +				__llist_add(&finish->link, &finish_passes);
> +			}
> +
> +		} else {
> +			ret = interval_sub->ops->invalidate(interval_sub,
> +							    &range,
> +							    cur_seq);
> +		}
>  		WARN_ON(!ret);
>  	}
>  
> +	mn_itree_finish_pass(&finish_passes);
>  	mn_itree_inv_end(subscriptions);
>  }
>  
> @@ -430,7 +456,9 @@ static int mn_itree_invalidate(struct mmu_notifier_subscriptions *subscriptions,
>  			       const struct mmu_notifier_range *range)
>  {
>  	struct mmu_interval_notifier *interval_sub;
> +	LLIST_HEAD(finish_passes);
>  	unsigned long cur_seq;
> +	int err = 0;
>  
>  	for (interval_sub =
>  		     mn_itree_inv_start_range(subscriptions, range, &cur_seq);
> @@ -438,23 +466,41 @@ static int mn_itree_invalidate(struct mmu_notifier_subscriptions *subscriptions,
>  	     interval_sub = mn_itree_inv_next(interval_sub, range)) {
>  		bool ret;
>  
> -		ret = interval_sub->ops->invalidate(interval_sub, range,
> -						    cur_seq);
> +		if (interval_sub->ops->invalidate_start) {
> +			struct mmu_interval_notifier_finish *finish = NULL;
> +
> +			ret = interval_sub->ops->invalidate_start(interval_sub,
> +								  range,
> +								  cur_seq,
> +								  &finish);
> +			if (ret && finish) {
> +				finish->notifier = interval_sub;
> +				__llist_add(&finish->link, &finish_passes);
> +			}
> +
> +		} else {
> +			ret = interval_sub->ops->invalidate(interval_sub,
> +							    range,
> +							    cur_seq);
> +		}
>  		if (!ret) {
>  			if (WARN_ON(mmu_notifier_range_blockable(range)))
>  				continue;
> -			goto out_would_block;
> +			err = -EAGAIN;
> +			break;
>  		}
>  	}
> -	return 0;
>  
> -out_would_block:
> +	mn_itree_finish_pass(&finish_passes);
> +
>  	/*
>  	 * On -EAGAIN the non-blocking caller is not allowed to call
>  	 * invalidate_range_end()
>  	 */
> -	mn_itree_inv_end(subscriptions);
> -	return -EAGAIN;
> +	if (err)
> +		mn_itree_inv_end(subscriptions);
> +
> +	return err;
>  }
>  
>  static int mn_hlist_invalidate_range_start(
> -- 
> 2.53.0
>