From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f50.google.com (mail-wg0-f50.google.com [74.125.82.50]) by kanga.kvack.org (Postfix) with ESMTP id D8B186B006E for ; Fri, 15 May 2015 05:15:04 -0400 (EDT) Received: by wgbhc8 with SMTP id hc8so72238870wgb.3 for ; Fri, 15 May 2015 02:15:04 -0700 (PDT) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id ei5si2631471wid.118.2015.05.15.02.15.02 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 15 May 2015 02:15:03 -0700 (PDT) Message-ID: <5555B914.8050800@suse.cz> Date: Fri, 15 May 2015 11:15:00 +0200 From: Vlastimil Babka MIME-Version: 1.0 Subject: Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse the page on WP fault References: <1429823043-157133-1-git-send-email-kirill.shutemov@linux.intel.com> <1429823043-157133-5-git-send-email-kirill.shutemov@linux.intel.com> In-Reply-To: <1429823043-157133-5-git-send-email-kirill.shutemov@linux.intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" , Andrew Morton , Andrea Arcangeli , Hugh Dickins Cc: Dave Hansen , Mel Gorman , Rik van Riel , Christoph Lameter , Naoya Horiguchi , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , Jerome Marchand , Sasha Levin , linux-kernel@vger.kernel.org, linux-mm@kvack.org On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote: > 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 > Tested-by: Sasha Levin Acked-by: Vlastimil Babka So couldn't the same trick be used in Patch 1 to avoid counting individual oder-0 pages? > --- > 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 0428e4c84e1d..17cdd6b9456b 100644 > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -524,7 +524,8 @@ static inline int page_swapcount(struct page *page) > 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 534f353e12bf..fd8af5b9917f 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1103,7 +1103,17 @@ 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 6dd365d1c488..3cd5f188b996 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -887,6 +887,9 @@ 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); > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org