From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754238AbaKSNBS (ORCPT ); Wed, 19 Nov 2014 08:01:18 -0500 Received: from mta-out1.inet.fi ([62.71.2.203]:58607 "EHLO kirsi1.inet.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751226AbaKSNBR (ORCPT ); Wed, 19 Nov 2014 08:01:17 -0500 Date: Wed, 19 Nov 2014 15:00:50 +0200 From: "Kirill A. Shutemov" To: Jerome Marchand Cc: "Kirill A. Shutemov" , Andrew Morton , Andrea Arcangeli , Dave Hansen , Hugh Dickins , Mel Gorman , Rik van Riel , Vlastimil Babka , Christoph Lameter , Naoya Horiguchi , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 06/19] mm: store mapcount for compound page separate Message-ID: <20141119130050.GA29884@node.dhcp.inet.fi> References: <1415198994-15252-1-git-send-email-kirill.shutemov@linux.intel.com> <1415198994-15252-7-git-send-email-kirill.shutemov@linux.intel.com> <546C761D.6050407@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <546C761D.6050407@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 19, 2014 at 11:51:09AM +0100, Jerome Marchand wrote: > On 11/05/2014 03:49 PM, Kirill A. Shutemov wrote: > > We're going to allow mapping of individual 4k pages of THP compound and > > we need a cheap way to find out how many time the compound page is > > mapped with PMD -- compound_mapcount() does this. > > > > page_mapcount() counts both: PTE and PMD mappings of the page. > > > > Signed-off-by: Kirill A. Shutemov > > --- > > include/linux/mm.h | 17 +++++++++++++++-- > > include/linux/rmap.h | 4 ++-- > > mm/huge_memory.c | 23 ++++++++++++++--------- > > mm/hugetlb.c | 4 ++-- > > mm/memory.c | 2 +- > > mm/migrate.c | 2 +- > > mm/page_alloc.c | 13 ++++++++++--- > > mm/rmap.c | 50 +++++++++++++++++++++++++++++++++++++++++++------- > > 8 files changed, 88 insertions(+), 27 deletions(-) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 1825c468f158..aef03acff228 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -435,6 +435,19 @@ static inline struct page *compound_head(struct page *page) > > return page; > > } > > > > +static inline atomic_t *compound_mapcount_ptr(struct page *page) > > +{ > > + return (atomic_t *)&page[1].mapping; > > +} > > IIUC your patch overloads the unused mapping field of the first tail > page to store the PMD mapcount. That's a non obvious trick. Why not make > it more explicit by adding a new field (say compound_mapcount - and the > appropriate comment of course) to the union to which mapping already belong? I don't think we want to bloat struct page description: nobody outside of helpers should use it direcly. And it's exactly what we did to store compound page destructor and compound page order. > The patch description would benefit from more explanation too. Agreed. -- Kirill A. Shutemov