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    SubjectRe: [PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for cow optimization
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    On Mon, Jul 29, 2013 at 1:28 PM, Joonsoo Kim <iamjoonsoo.kim@lge.com> 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 <mhocko@suse.cz>
    > Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
    > Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    >
    Acked-by: Hillf Danton <dhillf@gmail.com>

    > 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
    >


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