From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx137.postini.com [74.125.245.137]) by kanga.kvack.org (Postfix) with SMTP id ADBAF6B0032 for ; Wed, 17 Jul 2013 04:55:21 -0400 (EDT) Received: from /spool/local by e23smtp06.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 17 Jul 2013 18:47:35 +1000 Received: from d23relay05.au.ibm.com (d23relay05.au.ibm.com [9.190.235.152]) by d23dlp02.au.ibm.com (Postfix) with ESMTP id 220EF2BB0051 for ; Wed, 17 Jul 2013 18:55:13 +1000 (EST) Received: from d23av01.au.ibm.com (d23av01.au.ibm.com [9.190.234.96]) by d23relay05.au.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r6H8dlfq63307792 for ; Wed, 17 Jul 2013 18:39:48 +1000 Received: from d23av01.au.ibm.com (loopback [127.0.0.1]) by d23av01.au.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r6H8tAab002803 for ; Wed, 17 Jul 2013 18:55:11 +1000 Date: Wed, 17 Jul 2013 04:55:09 -0400 From: Wanpeng Li Subject: Re: [PATCH 6/9] mm, hugetlb: do not use a page in page cache for cow optimization Message-ID: <20130717085508.GA27397@hacker.(null)> Reply-To: Wanpeng Li References: <1373881967-16153-1-git-send-email-iamjoonsoo.kim@lge.com> <1373881967-16153-7-git-send-email-iamjoonsoo.kim@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1373881967-16153-7-git-send-email-iamjoonsoo.kim@lge.com> Sender: owner-linux-mm@kvack.org List-ID: To: Joonsoo Kim Cc: Andrew Morton , Rik van Riel , Mel Gorman , Michal Hocko , "Aneesh Kumar K.V" , KAMEZAWA Hiroyuki , Hugh Dickins , Davidlohr Bueso , David Gibson , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joonsoo Kim On Mon, Jul 15, 2013 at 06:52:44PM +0900, Joonsoo Kim wrote: >Currently, we use a page with mapped count 1 in page cache for cow >optimization. If we find this condition, we don't allocate a new >page and copy contents. Instead, we map this page directly. >This may introduce a problem that writting to private mapping overwrite >hugetlb file directly. You can find this situation with following code. > > size = 20 * MB; > flag = MAP_SHARED; > p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0); > if (p == MAP_FAILED) { > fprintf(stderr, "mmap() failed: %s\n", strerror(errno)); > return -1; > } > p[0] = 's'; > fprintf(stdout, "BEFORE STEAL PRIVATE WRITE: %c\n", p[0]); > munmap(p, size); > > flag = MAP_PRIVATE; > p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0); > if (p == MAP_FAILED) { > fprintf(stderr, "mmap() failed: %s\n", strerror(errno)); > } > p[0] = 'c'; > munmap(p, size); > > flag = MAP_SHARED; > p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0); > if (p == MAP_FAILED) { > fprintf(stderr, "mmap() failed: %s\n", strerror(errno)); > return -1; > } > fprintf(stdout, "AFTER STEAL PRIVATE WRITE: %c\n", p[0]); > munmap(p, size); > >We can see that "AFTER STEAL PRIVATE WRITE: c", not "AFTER STEAL >PRIVATE WRITE: s". If we turn off this optimization to a page >in page cache, the problem is disappeared. > >Signed-off-by: Joonsoo Kim > Good catch! Reviewed-by: Wanpeng Li >diff --git a/mm/hugetlb.c b/mm/hugetlb.c >index d4a1695..6c1eb9b 100644 >--- a/mm/hugetlb.c >+++ b/mm/hugetlb.c >@@ -2512,7 +2512,6 @@ static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma, > { > struct hstate *h = hstate_vma(vma); > struct page *old_page, *new_page; >- int avoidcopy; > int outside_reserve = 0; > unsigned long mmun_start; /* For mmu_notifiers */ > unsigned long mmun_end; /* For mmu_notifiers */ >@@ -2522,10 +2521,8 @@ static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma, > retry_avoidcopy: > /* If no-one else is actually using this page, avoid the copy > * and just make the page writable */ >- avoidcopy = (page_mapcount(old_page) == 1); >- if (avoidcopy) { >- if (PageAnon(old_page)) >- page_move_anon_rmap(old_page, vma, address); >+ if (page_mapcount(old_page) == 1 && PageAnon(old_page)) { >+ page_move_anon_rmap(old_page, vma, address); > set_huge_ptep_writable(vma, address, ptep); > return 0; > } >-- >1.7.9.5 > >-- >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 -- 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