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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

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@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            | 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
+ * @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..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