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Message-ID: <1a47196a82c4d0644e2dae3af5d3f9f33bfc8fd8.camel@linux.intel.com>
Date: Wed, 03 Sep 2025 14:56:49 +0200
From: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
To: intel-xe@...ts.freedesktop.org
Cc: Jason Gunthorpe <jgg@...pe.ca>, Andrew Morton
<akpm@...ux-foundation.org>, Simona Vetter <simona.vetter@...ll.ch>, Dave
Airlie <airlied@...il.com>, Alistair Popple <apopple@...dia.com>,
dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Matthew Brost <matthew.brost@...el.com>,
Christian König <christian.koenig@....com>
Subject: Re: [PATCH 1/6] mm/mmu_notifier: Allow two-pass struct
mmu_interval_notifiers
Hi,
@Jason, @Alistair, Gentle ping, could you have a look and R-B, Ack if
OK?
Thanks,
Thomas
On Thu, 2025-08-21 at 13:46 +0200, 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)
>
> Cc: Jason Gunthorpe <jgg@...pe.ca>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Simona Vetter <simona.vetter@...ll.ch>
> Cc: Dave Airlie <airlied@...il.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: <dri-devel@...ts.freedesktop.org>
> Cc: <linux-mm@...ck.org>
> Cc: <linux-kernel@...r.kernel.org>
>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> ---
> include/linux/mmu_notifier.h | 42 ++++++++++++++++++++++++
> mm/mmu_notifier.c | 63 ++++++++++++++++++++++++++++++----
> --
> 2 files changed, 96 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mmu_notifier.h
> b/include/linux/mmu_notifier.h
> index d1094c2d5fb6..14cfb3735699 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -233,16 +233,58 @@ 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
> + *
> + * 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 strct
> 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 list_head link;
> + /**
> + * @finish: Driver callback for the finish pass.
> + * @final: Pointer to the mmu_interval_notifier_finish
> structure.
> + * @range: The mmu_notifier_range.
> + * @cur_seq: The current sequence set by the first pass.
> + *
> + * Note that there is no error reporting for additional
> passes.
> + */
> + void (*finish)(struct mmu_interval_notifier_finish *final,
> + const struct mmu_notifier_range *range,
> + unsigned long cur_seq);
> +};
> +
> /**
> * 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 callto @invalidate_start
> is the first
> + * pass and any struct
> mmu_interval_notifier_finish pointer
> + * returned in the @fini parameter describes the
> final pass.
> + * If @fini is %NULL on return, then no final
> pass will be
> + * called.
> */
> 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
> **final);
> };
>
> struct mmu_interval_notifier {
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 8e0125dc0522..fceadcd8ca24 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -260,6 +260,18 @@ mmu_interval_read_begin(struct
> mmu_interval_notifier *interval_sub)
> }
> EXPORT_SYMBOL_GPL(mmu_interval_read_begin);
>
> +static void mn_itree_final_pass(struct list_head *final_passes,
> + const struct mmu_notifier_range
> *range,
> + unsigned long cur_seq)
> +{
> + struct mmu_interval_notifier_finish *f, *next;
> +
> + list_for_each_entry_safe(f, next, final_passes, link) {
> + list_del(&f->link);
> + f->finish(f, range, cur_seq);
> + }
> +}
> +
> static void mn_itree_release(struct mmu_notifier_subscriptions
> *subscriptions,
> struct mm_struct *mm)
> {
> @@ -271,6 +283,7 @@ static void mn_itree_release(struct
> mmu_notifier_subscriptions *subscriptions,
> .end = ULONG_MAX,
> };
> struct mmu_interval_notifier *interval_sub;
> + LIST_HEAD(final_passes);
> unsigned long cur_seq;
> bool ret;
>
> @@ -278,11 +291,25 @@ 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 *final =
> NULL;
> +
> + ret = interval_sub->ops-
> >invalidate_start(interval_sub,
> +
> &range,
> +
> cur_seq,
> +
> &final);
> + if (ret && final)
> + list_add_tail(&final->link,
> &final_passes);
> +
> + } else {
> + ret = interval_sub->ops-
> >invalidate(interval_sub,
> + &range,
> +
> cur_seq);
> + }
> WARN_ON(!ret);
> }
>
> + mn_itree_final_pass(&final_passes, &range, cur_seq);
> mn_itree_inv_end(subscriptions);
> }
>
> @@ -430,7 +457,9 @@ static int mn_itree_invalidate(struct
> mmu_notifier_subscriptions *subscriptions,
> const struct mmu_notifier_range
> *range)
> {
> struct mmu_interval_notifier *interval_sub;
> + LIST_HEAD(final_passes);
> unsigned long cur_seq;
> + int err = 0;
>
> for (interval_sub =
> mn_itree_inv_start_range(subscriptions, range,
> &cur_seq);
> @@ -438,23 +467,39 @@ 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 *final =
> NULL;
> +
> + ret = interval_sub->ops-
> >invalidate_start(interval_sub,
> +
> range,
> +
> cur_seq,
> +
> &final);
> + if (ret && final)
> + list_add_tail(&final->link,
> &final_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_final_pass(&final_passes, range, cur_seq);
> +
> /*
> * 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(
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