From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx168.postini.com [74.125.245.168]) by kanga.kvack.org (Postfix) with SMTP id 5218B6B0031 for ; Mon, 29 Jul 2013 05:33:29 -0400 (EDT) Received: by mail-ob0-f180.google.com with SMTP id up14so726615obb.25 for ; Mon, 29 Jul 2013 02:33:28 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1375075701-5998-7-git-send-email-iamjoonsoo.kim@lge.com> References: <1375075701-5998-1-git-send-email-iamjoonsoo.kim@lge.com> <1375075701-5998-7-git-send-email-iamjoonsoo.kim@lge.com> Date: Mon, 29 Jul 2013 17:33:28 +0800 Message-ID: Subject: Re: [PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for cow optimization From: Hillf Danton Content-Type: text/plain; charset=UTF-8 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 , Wanpeng Li , Naoya Horiguchi On Mon, Jul 29, 2013 at 1:28 PM, 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. > > So, I change the trigger condition of optimization. If this page is not > AnonPage, we don't do optimization. This makes this optimization turning > off for a page cache. > > Acked-by: Michal Hocko > Reviewed-by: Wanpeng Li > Reviewed-by: Naoya Horiguchi > Signed-off-by: Joonsoo Kim > Acked-by: Hillf Danton > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 2e52afea..1f6b3a6 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -2511,7 +2511,6 @@ 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 */ > @@ -2521,10 +2520,8 @@ 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