linux-kernel - [PATCH v2 1/2] mm/damon: validate if the pmd entry is present before accessing

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Message-Id: <58b1d1f5fbda7db49ca886d9ef6783e3dcbbbc98.1660805030.git.baolin.wang@linux.alibaba.com>
Date:   Thu, 18 Aug 2022 15:37:43 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     sj@...nel.org, akpm@...ux-foundation.org
Cc:     baolin.wang@...ux.alibaba.com, muchun.song@...ux.dev,
        mike.kravetz@...cle.com, damon@...ts.linux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] mm/damon: validate if the pmd entry is present before accessing

The pmd_huge() is used to validate if the pmd entry is mapped by a huge
page, also including the case of non-present (migration or hwpoisoned)
pmd entry on arm64 or x86 architectures. That means the pmd_pfn() can
not get the correct pfn number for the non-present pmd entry, which
will cause damon_get_page() to get an incorrect page struct (also
may be NULL by pfn_to_online_page()) to make the access statistics
incorrect.

Moreover it does not make sense that we still waste time to get the
page of the non-present entry, just treat it as not-accessed and skip it,
that keeps consistent with non-present pte level entry.

Thus adding a pmd entry present validation to fix above issues.

Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
Reviewed-by: SeongJae Park <sj@...nel.org>
---
Changes from v1:
 - Update the commit message to make it more clear.
 - Add reviewed tag from SeongJae.
---
 mm/damon/vaddr.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 3c7b9d6..1d16c6c 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -304,6 +304,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,

 	if (pmd_huge(*pmd)) {
 		ptl = pmd_lock(walk->mm, pmd);
+		if (!pmd_present(*pmd)) {
+			spin_unlock(ptl);
+			return 0;
+		}
+
 		if (pmd_huge(*pmd)) {
 			damon_pmdp_mkold(pmd, walk->mm, addr);
 			spin_unlock(ptl);
@@ -431,6 +436,11 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	if (pmd_huge(*pmd)) {
 		ptl = pmd_lock(walk->mm, pmd);
+		if (!pmd_present(*pmd)) {
+			spin_unlock(ptl);
+			return 0;
+		}
+
 		if (!pmd_huge(*pmd)) {
 			spin_unlock(ptl);
 			goto regular_page;
-- 
1.8.3.1