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

[Aneesh Kumar K.V]

Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> 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@xxxxxxx>
>
> 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

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