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

Thomas Hellström
2026-03-03T13:34:49Z

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.
+ * 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);
};

struct mmu_interval_notifier {
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index a6cdf3674bdc..4d8a64ce8eda 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(
@@ -976,6 +1022,7 @@ int mmu_interval_notifier_insert(struct mmu_interval_notifier *interval_sub,
struct mmu_notifier_subscriptions *subscriptions;
int ret;

+ WARN_ON_ONCE(ops->invalidate_start && !ops->invalidate_finish);
might_lock(&mm->mmap_lock);

subscriptions = smp_load_acquire(&mm->notifier_subscriptions);
--
2.53.0