From 37a2140dc2145a6f154172286944a1861e978dfd Mon Sep 17 00:00:00 2001 From: Joonsoo Kim Date: Wed, 11 Sep 2013 14:21:04 -0700 Subject: [PATCH] mm, hugetlb: do not use a page in page cache for cow optimization 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. Signed-off-by: Joonsoo Kim Acked-by: Michal Hocko Reviewed-by: Wanpeng Li Reviewed-by: Naoya Horiguchi Cc: Aneesh Kumar K.V Acked-by: Hillf Danton Cc: Rik van Riel Cc: Mel Gorman Cc: "Aneesh Kumar K.V" Cc: KAMEZAWA Hiroyuki Cc: Hugh Dickins Cc: Davidlohr Bueso Cc: David Gibson Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/hugetlb.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index a13be48b818b..da027a3307af 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -2528,7 +2528,6 @@ 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 */ @@ -2538,10 +2537,8 @@ 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; } -- 2.30.2