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From:   Yin Fengwei <fengwei.yin@intel.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, akpm@linux-foundation.org,
        willy@infradead.org, vishal.moola@gmail.com,
        wangkefeng.wang@huawei.com, minchan@kernel.org, yuzhao@google.com,
        david@redhat.com, ryan.roberts@arm.com, shy828301@gmail.com
Cc:     fengwei.yin@intel.com
Subject: [PATCH v2 3/3] madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check
Date:   Tue,  8 Aug 2023 10:09:17 +0800
Message-Id: <20230808020917.2230692-4-fengwei.yin@intel.com>
In-Reply-To: <20230808020917.2230692-1-fengwei.yin@intel.com>
References: <20230808020917.2230692-1-fengwei.yin@intel.com>
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Commit 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a
folio") replaced the page_mapcount() with folio_mapcount() to check
whether the folio is shared by other mapping.

It's not correct for large folios. 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: 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a folio")
Cc: stable@vger.kernel.org
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Reviewed-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
---
 mm/madvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 49af35e2d99a..4dded5d27e7e 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -683,7 +683,7 @@ 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