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    Subject[PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for cow optimization
    Date
    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>

    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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