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Date: Wed, 27 Mar 2024 14:31:00 -0700
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Subject: [RFC PATCH v3 1/8] mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true
From: Yuanchu Xie <yuanchu@google.com>
To: David Hildenbrand <david@redhat.com>, "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, 
	Khalid Aziz <khalid.aziz@oracle.com>, Henry Huang <henry.hj@antgroup.com>, 
	Yu Zhao <yuzhao@google.com>, Dan Williams <dan.j.williams@intel.com>, 
	Gregory Price <gregory.price@memverge.com>, Huang Ying <ying.huang@intel.com>
Cc: Wei Xu <weixugc@google.com>, David Rientjes <rientjes@google.com>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "Rafael J. Wysocki" <rafael@kernel.org>, 
	Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, 
	Michal Hocko <mhocko@kernel.org>, Roman Gushchin <roman.gushchin@linux.dev>, 
	Muchun Song <muchun.song@linux.dev>, Shuah Khan <shuah@kernel.org>, 
	Yosry Ahmed <yosryahmed@google.com>, Matthew Wilcox <willy@infradead.org>, 
	Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>, Kairui Song <kasong@tencent.com>, 
	"Michael S. Tsirkin" <mst@redhat.com>, Vasily Averin <vasily.averin@linux.dev>, Nhat Pham <nphamcs@gmail.com>, 
	Miaohe Lin <linmiaohe@huawei.com>, Qi Zheng <zhengqi.arch@bytedance.com>, 
	Abel Wu <wuyun.abel@bytedance.com>, "Vishal Moola (Oracle)" <vishal.moola@gmail.com>, 
	Kefeng Wang <wangkefeng.wang@huawei.com>, Yuanchu Xie <yuanchu@google.com>, 
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
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When non-leaf pmd accessed bits are available, MGLRU page table walks
can clear the accessed bit and promptly ignore the accessed bit on the
pte because it's on a different node, so the walk does not update the
generation of said page. When the next scan comes around on the right
node, 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 measuring the
working set size of a node/memcg.

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@google.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;
 
-- 
2.44.0.396.g6e790dbe36-goog