Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751765Ab3GaFl4 (ORCPT ); Wed, 31 Jul 2013 01:41:56 -0400 Received: from LGEMRELSE7Q.lge.com ([156.147.1.151]:54583 "EHLO LGEMRELSE7Q.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750839Ab3GaFlz (ORCPT ); Wed, 31 Jul 2013 01:41:55 -0400 X-AuditID: 9c930197-b7bfbae000000e88-a5-51f8a3a2f9de Date: Wed, 31 Jul 2013 14:41:55 +0900 From: Joonsoo Kim 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 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> User-Agent: Mutt/1.5.21 (2010-09-15) X-Brightmail-Tracker: AAAAAA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2249 Lines: 59 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/