linux-kernel - [PATCHv9 04/36] mm, thp: adjust conditions when we can reuse the page on WP fault

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Message-Id: <1437402069-105900-5-git-send-email-kirill.shutemov@linux.intel.com>
Date:	Mon, 20 Jul 2015 17:20:37 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Cc:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv9 04/36] 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@...ux.intel.com>
Tested-by: Sasha Levin <sasha.levin@...cle.com>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Acked-by: Jerome Marchand <jmarchan@...hat.com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
---
 include/linux/swap.h |  3 ++-
 mm/huge_memory.c     | 12 +++++++++++-
 mm/swapfile.c        |  3 +++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index ec62fcbfcad3..24ad6c3b0853 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -537,7 +537,8 @@ static inline int swp_swapcount(swp_entry_t entry)
 	return 0;
 }
 
-#define reuse_swap_page(page)	(page_mapcount(page) == 1)
+#define reuse_swap_page(page) \
+	(!PageTransCompound(page) && page_mapcount(page) == 1)
 
 static inline int try_to_free_swap(struct page *page)
 {
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6fd6df16e55d..8d5a8881c60e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1171,7 +1171,17 @@ 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);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 866c982cea9b..89ea6af27dc0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -929,6 +929,9 @@ int reuse_swap_page(struct page *page)
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	if (unlikely(PageKsm(page)))
 		return 0;
+	/* The page is part of THP and cannot be reused */
+	if (PageTransCompound(page))
+		return 0;
 	count = page_mapcount(page);
 	if (count <= 1 && PageSwapCache(page)) {
 		count += page_swapcount(page);
-- 
2.1.4

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