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Message-ID: <20130731054154.GN2548@lge.com>
Date: Wed, 31 Jul 2013 14:41:55 +0900
From: Joonsoo Kim <iamjoonsoo.kim@....com>
To: David Gibson <david@...son.dropbear.id.au>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
Michal Hocko <mhocko@...e.cz>,
"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Hugh Dickins <hughd@...gle.com>,
Davidlohr Bueso <davidlohr.bueso@...com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for
cow optimization
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
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