linux-kernel - [PATCH 2/2] madvise: don't use mapcount() against large folio for sharing check

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Date:   Sat, 29 Jul 2023 00:13:56 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, akpm@...ux-foundation.org,
        willy@...radead.org, vishal.moola@...il.com,
        wangkefeng.wang@...wei.com, minchan@...nel.org, yuzhao@...gle.com,
        david@...hat.com, ryan.roberts@....com, shy828301@...il.com
Cc:     fengwei.yin@...el.com
Subject: [PATCH 2/2] madvise: don't use mapcount() against large folio for sharing check

The commits
98b211d6415f ("madvise: convert madvise_free_pte_range() to use
a folio")
fc986a38b670 ("mm: huge_memory: convert madvise_free_huge_pmd to
use a folio")

replaced the page_mapcount() with folio_mapcount() to check whether
the folio is shared by other mapping.

But it's not correct for large folio. folio_mapcount() returns the
total mapcount of large folio which is not suitable to detect whether
the folio is shared.

Use folio_estimated_sharers() which returns a estimated number of
shares. That means it's not 100% correct. But it should be OK for
madvise case here.

Fixes: 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a folio")
Fixes: fc986a38b670 ("mm: huge_memory: convert madvise_free_huge_pmd to use a folio")
Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
Reviewed-by: Yu Zhao <yuzhao@...gle.com>
---
 mm/huge_memory.c | 2 +-
 mm/madvise.c     | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index eb3678360b97..68c890875257 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1613,7 +1613,7 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	 * If other processes are mapping this folio, we couldn't discard
 	 * the folio unless they all do MADV_FREE so let's skip the folio.
 	 */
-	if (folio_mapcount(folio) != 1)
+	if (folio_estimated_sharers(folio) != 1)
 		goto out;
 
 	if (!folio_trylock(folio))
diff --git a/mm/madvise.c b/mm/madvise.c
index 148b46beb039..55bdf641abfa 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -678,7 +678,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 		if (folio_test_large(folio)) {
 			int err;
 
-			if (folio_mapcount(folio) != 1)
+			if (folio_estimated_sharers(folio) != 1)
 				break;
 			if (!folio_trylock(folio))
 				break;
-- 
2.39.2