+ mm-multi-gen-lru-ignore-non-leaf-pmd_young-for-force_scan=true.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true
has been added to the -mm mm-unstable branch.  Its filename is
     mm-multi-gen-lru-ignore-non-leaf-pmd_young-for-force_scan=true.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-multi-gen-lru-ignore-non-leaf-pmd_young-for-force_scan=true.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Yuanchu Xie <yuanchu@google.com>
Subject: mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true
Date: Tue, 13 Aug 2024 09:37:59 -0700

When non-leaf pmd accessed bits are available, MGLRU page table walks can
clear the non-leaf pmd accessed bit and ignore the accessed bit on the pte
if it's on a different node, skipping a generation update as well.  If
another scan occurs on the same node as said skipped pte.

The non-leaf pmd accessed bit might remain cleared and the pte accessed
bits won't be checked.  While this is sufficient for reclaim-driven aging,
where the goal is to select a reasonably cold page, the access can be
missed when aging proactively for workingset estimation of a node/memcg.

In more detail, get_pfn_folio returns NULL if the folio's nid != node
under scanning, so the page table walk skips processing of said pte.  Now
the pmd_young flag on this pmd is cleared, and if none of the pte's are
accessed before another scan occurs on the folio's node, the pmd_young
check fails and the pte accessed bit is skipped.

Since force_scan disables various other optimizations, we check force_scan
to ignore the non-leaf pmd accessed bit.

Link: https://lkml.kernel.org/r/20240813163759.742675-1-yuanchu@google.com
Signed-off-by: Yuanchu Xie <yuanchu@google.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmscan.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c~mm-multi-gen-lru-ignore-non-leaf-pmd_young-for-force_scan=true
+++ a/mm/vmscan.c
@@ -3499,7 +3499,7 @@ static void walk_pmd_range_locked(pud_t
 			goto next;
 
 		if (!pmd_trans_huge(pmd[i])) {
-			if (should_clear_pmd_young())
+			if (!walk->force_scan && should_clear_pmd_young())
 				pmdp_test_and_clear_young(vma, addr, pmd + i);
 			goto next;
 		}
@@ -3586,7 +3586,7 @@ restart:
 
 		walk->mm_stats[MM_NONLEAF_TOTAL]++;
 
-		if (should_clear_pmd_young()) {
+		if (!walk->force_scan && should_clear_pmd_young()) {
 			if (!pmd_young(val))
 				continue;
 
_

Patches currently in -mm which might be from yuanchu@google.com are

mm-multi-gen-lru-ignore-non-leaf-pmd_young-for-force_scan=true.patch