On 02.04.24 01:29, James Houghton wrote:
The bitmap is provided for secondary MMUs to use if they support it. For
test_young(), after it returns, the bitmap represents the pages that
were young in the interval [start, end). For clear_young, it represents
the pages that we wish the secondary MMU to clear the accessed/young bit
for.
If a bitmap is not provided, the mmu_notifier_{test,clear}_young() API
should be unchanged except that if young PTEs are found and the
architecture supports passing in a bitmap, instead of returning 1,
MMU_NOTIFIER_YOUNG_FAST is returned.
This allows MGLRU's look-around logic to work faster, resulting in a 4%
improvement in real workloads[1]. Also introduce MMU_NOTIFIER_YOUNG_FAST
to indicate to main mm that doing look-around is likely to be
beneficial.
If the secondary MMU doesn't support the bitmap, it must return
an int that contains MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.
[1]: https://lore.kernel.org/all/20230609005935.42390-1-yuzhao@xxxxxxxxxx/
Suggested-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
include/linux/mmu_notifier.h | 93 +++++++++++++++++++++++++++++++++---
include/trace/events/kvm.h | 13 +++--
mm/mmu_notifier.c | 20 +++++---
virt/kvm/kvm_main.c | 19 ++++++--
4 files changed, 123 insertions(+), 22 deletions(-)
diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index f349e08a9dfe..daaa9db625d3 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -61,6 +61,10 @@ enum mmu_notifier_event {
#define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
+#define MMU_NOTIFIER_YOUNG (1 << 0)
+#define MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE (1 << 1)
Especially this one really deserves some documentation :)
+#define MMU_NOTIFIER_YOUNG_FAST (1 << 2)
And that one as well.
Likely best to briefly document all of them, and how they are
supposed to be used (return value for X).
+
struct mmu_notifier_ops {
/*
* Called either by mmu_notifier_unregister or when the mm is
@@ -106,21 +110,36 @@ 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.
+ *
+ * If @bitmap is given but is not supported, return
+ * MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.
+ *
+ * If the walk is done "quickly" and there were young PTEs,
+ * MMU_NOTIFIER_YOUNG_FAST is returned.
*/
int (*clear_young)(struct mmu_notifier *subscription,
struct mm_struct *mm,
unsigned long start,
- unsigned long end);
+ unsigned long end,
+ unsigned long *bitmap);
/*
* test_young is called to check the young/accessed bitflag in
* 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.
+ *
+ * If @bitmap is given but is not supported, return
+ * MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE.
+ *
+ * If the walk is done "quickly" and there were young PTEs,
+ * MMU_NOTIFIER_YOUNG_FAST is returned.
*/
int (*test_young)(struct mmu_notifier *subscription,
struct mm_struct *mm,
- unsigned long address);
+ unsigned long start,
+ unsigned long end,
+ unsigned long *bitmap);
What does "quickly" mean (why not use "fast")? What are the semantics, I
don't find any existing usage of that in this file.
Further, what is MMU_NOTIFIER_YOUNG you introduce used for?
--
Cheers,
David / dhildenb
