From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx166.postini.com [74.125.245.166]) by kanga.kvack.org (Postfix) with SMTP id BD2276B0031 for ; Wed, 31 Jul 2013 01:41:55 -0400 (EDT) Date: Wed, 31 Jul 2013 14:41:55 +0900 From: Joonsoo Kim Subject: Re: [PATCH v3 6/9] mm, hugetlb: do not use a page in page cache for cow optimization Message-ID: <20130731054154.GN2548@lge.com> References: <1375075701-5998-1-git-send-email-iamjoonsoo.kim@lge.com> <1375075701-5998-7-git-send-email-iamjoonsoo.kim@lge.com> <20130729223708.GG29970@voom.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130729223708.GG29970@voom.fritz.box> Sender: owner-linux-mm@kvack.org List-ID: To: David Gibson Cc: Andrew Morton , Rik van Riel , Mel Gorman , Michal Hocko , "Aneesh Kumar K.V" , KAMEZAWA Hiroyuki , Hugh Dickins , Davidlohr Bueso , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Wanpeng Li , Naoya Horiguchi , Hillf Danton 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org