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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 2/3] madvise:madvise_free_huge_pmd(): don't use mapcount() against large folio for sharing check
Date:   Tue,  8 Aug 2023 10:09:16 +0800
Message-Id: <20230808020917.2230692-3-fengwei.yin@intel.com>
In-Reply-To: <20230808020917.2230692-1-fengwei.yin@intel.com>
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Commit 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.

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: fc986a38b670 ("mm: huge_memory: convert madvise_free_huge_pmd 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/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 154c210892a1..0b709d2c46c6 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1612,7 +1612,7 @@ 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))
-- 
2.39.2