madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharin...

commit 0e0e9bd5f7b9d40fd03b70092367247d52da1db0 upstream.

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.

Link: https://lkml.kernel.org/r/20230808020917.2230692-4-fengwei.yin@intel.com
Fixes: 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a folio")
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Reviewed-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f83a1c9ec8e46d13bf5c0b3947e40164bafc66a9..104ec00823da8735b22e9c0ce637f3202207516b 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1727,6 +1727,25 @@ static inline size_t folio_size(struct folio *folio)
        return PAGE_SIZE << folio_order(folio);
 }
 
+/**
+ * folio_estimated_sharers - Estimate the number of sharers of a folio.
+ * @folio: The folio.
+ *
+ * folio_estimated_sharers() aims to serve as a function to efficiently
+ * estimate the number of processes sharing a folio. This is done by
+ * looking at the precise mapcount of the first subpage in the folio, and
+ * assuming the other subpages are the same. This may not be true for large
+ * folios. If you want exact mapcounts for exact calculations, look at
+ * page_mapcount() or folio_total_mapcount().
+ *
+ * Return: The estimated number of processes sharing a folio.
+ */
+static inline int folio_estimated_sharers(struct folio *folio)
+{
+       return page_mapcount(folio_page(folio, 0));
+}
+
+
 #ifndef HAVE_ARCH_MAKE_PAGE_ACCESSIBLE
 static inline int arch_make_page_accessible(struct page *page)
 {

diff --git a/mm/madvise.c b/mm/madvise.c
index d03e149ffe6e86af9aaf6d55d4bbf374d915c026..5973399b2f9b7c8ae14b4213f85847cda2f647b5 100644 (file)
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -654,8 +654,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
                 * deactivate all pages.
                 */
                if (folio_test_large(folio)) {
-                       if (folio_mapcount(folio) != 1)
-                               goto out;
+                       if (folio_estimated_sharers(folio) != 1)
+                               break;
                        folio_get(folio);
                        if (!folio_trylock(folio)) {
                                folio_put(folio);