Messages in this thread Patch in this message | | | From | Joonsoo Kim <> | Subject | [PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for cow optimization | Date | Mon, 29 Jul 2013 14:28:18 +0900 |
| |
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
| |