linux-kernel - [PATCH v2 12/14] mm: multi-gen LRU: walk_pte_range() use pte_offset_map_rw

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Message-Id: <0e2980e9a31d3503a5dfb350c04f4ede4d05220e.1724310149.git.zhengqi.arch@bytedance.com>
Date: Thu, 22 Aug 2024 15:13:27 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: david@...hat.com,
	hughd@...gle.com,
	willy@...radead.org,
	muchun.song@...ux.dev,
	vbabka@...nel.org,
	akpm@...ux-foundation.org,
	rppt@...nel.org,
	vishal.moola@...il.com,
	peterx@...hat.com,
	ryan.roberts@....com,
	christophe.leroy2@...soprasteria.com
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH v2 12/14] mm: multi-gen LRU: walk_pte_range() use pte_offset_map_rw_nolock()

In walk_pte_range(), we may modify the pte entry after holding the ptl, so
convert it to using pte_offset_map_rw_nolock(). At this time, the write
lock of mmap_lock is not held, and the pte_same() check is not performed
after the ptl held, so we should get pmdval and do pmd_same() check to
ensure the stability of pmd entry.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/vmscan.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5dc96a843466a..89ef1ac8eb1a6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3396,8 +3396,10 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
 	struct pglist_data *pgdat = lruvec_pgdat(walk->lruvec);
 	DEFINE_MAX_SEQ(walk->lruvec);
 	int old_gen, new_gen = lru_gen_from_seq(max_seq);
+	pmd_t pmdval;
 
-	pte = pte_offset_map_nolock(args->mm, pmd, start & PMD_MASK, &ptl);
+	pte = pte_offset_map_rw_nolock(args->mm, pmd, start & PMD_MASK, &pmdval,
+				       &ptl);
 	if (!pte)
 		return false;
 	if (!spin_trylock(ptl)) {
@@ -3405,6 +3407,11 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
 		return false;
 	}
 
+	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
+		pte_unmap_unlock(pte, ptl);
+		return false;
+	}
+
 	arch_enter_lazy_mmu_mode();
 restart:
 	for (i = pte_index(start), addr = start; addr != end; i++, addr += PAGE_SIZE) {
-- 
2.20.1