linux-kernel - [PATCH v2 1/3] madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check

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Date:   Tue,  8 Aug 2023 10:09:15 +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 v2 1/3] madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check

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.

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. It should be OK for madvise case here.

User-visible effects is that the THP is skipped when user call madvise.
But the correct behavior is THP should be split and processed then.

NOTE: this change is a temporary fix to reduce the user-visible effects
before the long term fix from David is ready.

Fixes: 07e8c82b5eff ("madvise: convert madvise_cold_or_pageout_pte_range() to use folios")
Cc: stable@...r.kernel.org
Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
Reviewed-by: Yu Zhao <yuzhao@...gle.com>
Reviewed-by: Ryan Roberts <ryan.roberts@....com>
---
 mm/madvise.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 1d7933933e31..49af35e2d99a 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;
-- 
2.39.2