linux-kernel - [RFC PATCH v2 1/4] madvise: not use mapcount() against large folio for sharing check

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Date:   Fri, 21 Jul 2023 17:40:40 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, minchan@...nel.org, yuzhao@...gle.com,
        willy@...radead.org, david@...hat.com, ryan.roberts@....com,
        shy828301@...il.com
Cc:     fengwei.yin@...el.com
Subject: [RFC PATCH v2 1/4] madvise: not use mapcount() against large folio for sharing check

The commit
07e8c82b5eff ("madvise: convert madvise_cold_or_pageout_pte_range() to
use folios") 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.

Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
---
 mm/madvise.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 38382a5d1e39..f12933ebcc24 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -383,7 +383,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		folio = pfn_folio(pmd_pfn(orig_pmd));
 
 		/* Do not interfere with other mappings of this folio */
-		if (folio_mapcount(folio) != 1)
+		if (folio_estimated_sharers(folio) != 1)
 			goto huge_unlock;
 
 		if (pageout_anon_only_filter && !folio_test_anon(folio))
@@ -459,7 +459,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		if (folio_test_large(folio)) {
 			int err;
 
-			if (folio_mapcount(folio) != 1)
+			if (folio_estimated_sharers(folio) != 1)
 				break;
 			if (pageout_anon_only_filter && !folio_test_anon(folio))
 				break;
@@ -682,7 +682,7 @@ 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