mm/mmu_notifier: Allow two-pass struct mmu_interval_notifiers

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)
v4:
- Addressed documentation review comments by David Hildenbrand.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
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>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patch.msgid.link/20260305093909.43623-2-thomas.hellstrom@linux.intel.com

1 files changed, 42 insertions, 0 deletions

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index d1094c2d5fb6..b60673a8e0bb 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -234,15 +234,57 @@ struct mmu_notifier {
 };
 
 /**
+ * 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 start pass.
+ * 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 start and the finish 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.
+ *
+ * If allocation fails, then the &mmu_interval_notifier_ops->invalidate_start op
+ * needs to implements the full notifier functionality. Please refer to its
+ * documentation.
+ */
+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 finish pass.
+ *                    If *@finish is %NULL on return, then no final pass will be
+ *                    called, and @invalidate_start needs to implement the full
+ *                    notifier, behaving like @invalidate. The value of *@finish
+ *                    is guaranteed to be %NULL at function entry.
+ * @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 {