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

Yin Fengwei
2023-07-28T16:16:07Z

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@intel.com>
Reviewed-by: Yu Zhao <yuzhao@google.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