Re: [PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for cow optimization

Joonsoo Kim
2013-07-31T05:41:56Z

On Tue, Jul 30, 2013 at 08:37:08AM +1000, David Gibson wrote:
> On Mon, Jul 29, 2013 at 02:28:18PM +0900, 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.
>
> Please add this testcase to libhugetlbfs as well.

Okay!

Thanks.

>
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson