Re: [PATCH v6 05/11] mm: Add fast_only bool to test_young and clear_young MMU notifiers

[James Houghton <jthoughton@xxxxxxxxxx>]

On Thu, Aug 1, 2024 at 2:36 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 24.07.24 03:10, James Houghton wrote:
> > For implementers, the fast_only bool indicates that the age information
> > needs to be harvested such that we do not slow down other MMU operations,
> > and ideally that we are not ourselves slowed down by other MMU
> > operations.  Usually this means that the implementation should be
> > lockless.
>
> But what are the semantics if "fast_only" cannot be achieved by the
> implementer?
>
> Can we add some documentation to the new functions that explain what
> this mysterious "fast_only" is and what the expected semantics are?
> Please? :)

Thanks for pointing out the missing documentation. How's this?

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index 45c5995ebd84..c21992036dd3 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -106,6 +106,18 @@ struct mmu_notifier_ops {
         * clear_young is a lightweight version of clear_flush_young. Like the
         * latter, it is supposed to test-and-clear the young/accessed bitflag
         * in the secondary pte, but it may omit flushing the secondary tlb.
+        *
+        * The fast_only parameter indicates that this call should not block,
+        * and this function should not cause other MMU notifier calls to
+        * block. Usually this means that the implementation should be
+        * lockless.
+        *
+        * When called with fast_only, this notifier will be a no-op unless
+        * has_fast_aging is set on the struct mmu_notifier.
+        *
+        * When fast_only is true, if the implementer cannot determine that a
+        * range is young without blocking, it should return 0 (i.e.,
+        * that the range is NOT young).
         */
        int (*clear_young)(struct mmu_notifier *subscription,
                           struct mm_struct *mm,
@@ -118,6 +130,8 @@ struct mmu_notifier_ops {
         * the secondary pte. This is used to know if the page is
         * frequently used without actually clearing the flag or tearing
         * down the secondary mapping on the page.
+        *
+        * The fast_only parameter has the same meaning as with clear_young.
         */
        int (*test_young)(struct mmu_notifier *subscription,
                          struct mm_struct *mm,

I've also moved the commit that follows this one (the one that adds
has_fast_aging) to be before this one so that the comment makes sense.