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Date: Tue, 23 Jul 2013 00:26:07 -0400 From: Naoya Horiguchi <n-horiguchi@...jp.nec.com> To: Joonsoo Kim <iamjoonsoo.kim@....com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>, "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Hugh Dickins <hughd@...gle.com>, Davidlohr Bueso <davidlohr.bueso@...com>, David Gibson <david@...son.dropbear.id.au>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Joonsoo Kim <js1304@...il.com> Subject: Re: [PATCH v2 07/10] mm, hugetlb: do not use a page in page cache for cow optimization On Mon, Jul 22, 2013 at 05:36:28PM +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. Looks good to me. Thanks for the fix. Reviewed-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com> > Reviewed-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 7ca8733..8a61638 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -2508,7 +2508,6 @@ 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 */ > @@ -2518,10 +2517,8 @@ 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@...ck.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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