On 02.08.24 01:13, James Houghton wrote:
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.
+ *
Probably makes sense to highlight the parameters like @fast_only
+ * 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.
"... and will return 0 (NOT young)." ?
+ *
+ * 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.
Makes sense, thanks!
--
Cheers,
David / dhildenb
