mm, thp: adjust conditions when we can reuse the page on WP fault

With new refcounting we will be able map the same compound page with
PTEs and PMDs.  It requires adjustment to conditions when we can reuse
the page on write-protection fault.

For PTE fault we can't reuse the page if it's part of huge page.

For PMD we can only reuse the page if nobody else maps the huge page or
it's part.  We can do it by checking page_mapcount() on each sub-page,
but it's expensive.

The cheaper way is to check page_count() to be equal 1: every mapcount
takes page reference, so this way we can guarantee, that the PMD is the
only mapping.

This approach can give false negative if somebody pinned the page, but
that doesn't affect correctness.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 1 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4211682f223b..e45918d058b9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1199,7 +1199,17 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	page = pmd_page(orig_pmd);
 	VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
-	if (page_mapcount(page) == 1) {
+	/*
+	 * We can only reuse the page if nobody else maps the huge page or it's
+	 * part. We can do it by checking page_mapcount() on each sub-page, but
+	 * it's expensive.
+	 * The cheaper way is to check page_count() to be equal 1: every
+	 * mapcount takes page reference reference, so this way we can
+	 * guarantee, that the PMD is the only mapping.
+	 * This can give false negative if somebody pinned the page, but that's
+	 * fine.
+	 */
+	if (page_mapcount(page) == 1 && page_count(page) == 1) {
 		pmd_t entry;
 		entry = pmd_mkyoung(orig_pmd);
 		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);