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

Aneesh Kumar K.V
2013-07-15T13:55:54Z

Joonsoo Kim <iamjoonsoo.kim@lge.com> writes:

> 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.

Do we need to trun of the optimization for page cache completely ?
Can't we check for MAP_PRIVATE case ?

Also, we may want to add the above test into libhugetlbfs

>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
> 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;
> }

-aneesh