linux-kernel - [PATCH v1 1/7] mm: multi-gen LRU: ignore non-leaf pmd_young for force

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Date: Sat,  4 May 2024 00:30:05 -0700
From: Yuanchu Xie <yuanchu@...gle.com>
To: David Hildenbrand <david@...hat.com>, "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>, 
	Khalid Aziz <khalid.aziz@...cle.com>, Henry Huang <henry.hj@...group.com>, 
	Yu Zhao <yuzhao@...gle.com>, Dan Williams <dan.j.williams@...el.com>, 
	Gregory Price <gregory.price@...verge.com>, Huang Ying <ying.huang@...el.com>
Cc: Kalesh Singh <kaleshsingh@...gle.com>, Wei Xu <weixugc@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Shuah Khan <shuah@...nel.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Matthew Wilcox <willy@...radead.org>, Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>, 
	Kairui Song <kasong@...cent.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Vasily Averin <vasily.averin@...ux.dev>, Nhat Pham <nphamcs@...il.com>, 
	Miaohe Lin <linmiaohe@...wei.com>, Qi Zheng <zhengqi.arch@...edance.com>, 
	Abel Wu <wuyun.abel@...edance.com>, "Vishal Moola (Oracle)" <vishal.moola@...il.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Yuanchu Xie <yuanchu@...gle.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: [PATCH v1 1/7] mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true

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 occurrs 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 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 occurrs 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.

Signed-off-by: Yuanchu Xie <yuanchu@...gle.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4f9c854ce6cc..1a7c7d537db6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3522,7 +3522,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,

 		walk->mm_stats[MM_NONLEAF_TOTAL]++;

-		if (should_clear_pmd_young()) {
+		if (!walk->force_scan && should_clear_pmd_young()) {
 			if (!pmd_young(val))
 				continue;

-- 
2.45.0.rc1.225.g2a3ae87e7f-goog