Re: [PATCH v2 07/10] mm, hugetlb: do not use a page in page cache for cow optimization

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Tue, Jul 23, 2013 at 01:45:50PM +0200, Michal Hocko wrote:
> On Mon 22-07-13 17:36:28, Joonsoo Kim wrote:
> > 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.
> 
> It would be nice to describe the fix here as well. It is far from being
> intuitive and trivial.

Okay. I will describe how I fix the problem in all patches you pointed out.

Thanks for reviewing!

> 
> The fix seems to be correct.
> 
> > Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.cz>
> 
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 7ca8733..8a61638 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2508,7 +2508,6 @@ 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 */
> > @@ -2518,10 +2517,8 @@ 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
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs
> 
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