Commit 1f25fe20 authored Jan 15, 2016 by Kirill A. Shutemov Committed by Linus Torvalds Jan 15, 2016

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>

parent f627c2f5

include/linux/swap.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -538,7 +538,8 @@ 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)
		{

mm/huge_memory.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -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);

mm/swapfile.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -926,6 +926,9 @@ 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);

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