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

Joonsoo Kim
2013-07-15T10:08:13Z

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.

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;
}
--
1.7.9.5