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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240813163759.742675-1-yuanchu@google.com>
Date: Tue, 13 Aug 2024 09:37:59 -0700
From: Yuanchu Xie <yuanchu@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Lance Yang <ioworker0@...il.com>, 
	Huang Ying <ying.huang@...el.com>, Yu Zhao <yuzhao@...gle.com>
Cc: Yuanchu Xie <yuanchu@...gle.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH] 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 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.

Signed-off-by: Yuanchu Xie <yuanchu@...gle.com>
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cfa839284b92..4a112c2d1a64 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3476,7 +3476,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area
 			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;
 		}
@@ -3563,7 +3563,7 @@ 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;

-- 
2.46.0.76.ge559c4bf1a-goog