Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751771AbaFWFjm (ORCPT ); Mon, 23 Jun 2014 01:39:42 -0400 Received: from cn.fujitsu.com ([59.151.112.132]:21622 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1751491AbaFWFjl (ORCPT ); Mon, 23 Jun 2014 01:39:41 -0400 X-IronPort-AV: E=Sophos;i="5.00,758,1396972800"; d="scan'208";a="32269592" Message-ID: <53A7BD91.8020802@cn.fujitsu.com> Date: Mon, 23 Jun 2014 13:39:29 +0800 From: Zhang Yanfei User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20131030 Thunderbird/17.0.10 MIME-Version: 1.0 To: "Kirill A. Shutemov" CC: Vlastimil Babka , , Andrew Morton , David Rientjes , Minchan Kim , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel , Subject: Re: [PATCH v3 01/13] mm, THP: don't hold mmap_sem in khugepaged when allocating THP References: <1403279383-5862-1-git-send-email-vbabka@suse.cz> <1403279383-5862-2-git-send-email-vbabka@suse.cz> <20140620174533.GA9635@node.dhcp.inet.fi> In-Reply-To: <20140620174533.GA9635@node.dhcp.inet.fi> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.167.225.89] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello On 06/21/2014 01:45 AM, Kirill A. Shutemov wrote: > On Fri, Jun 20, 2014 at 05:49:31PM +0200, Vlastimil Babka wrote: >> When allocating huge page for collapsing, khugepaged currently holds mmap_sem >> for reading on the mm where collapsing occurs. Afterwards the read lock is >> dropped before write lock is taken on the same mmap_sem. >> >> Holding mmap_sem during whole huge page allocation is therefore useless, the >> vma needs to be rechecked after taking the write lock anyway. Furthemore, huge >> page allocation might involve a rather long sync compaction, and thus block >> any mmap_sem writers and i.e. affect workloads that perform frequent m(un)map >> or mprotect oterations. >> >> This patch simply releases the read lock before allocating a huge page. It >> also deletes an outdated comment that assumed vma must be stable, as it was >> using alloc_hugepage_vma(). This is no longer true since commit 9f1b868a13 >> ("mm: thp: khugepaged: add policy for finding target node"). > > There is no point in touching ->mmap_sem in khugepaged_alloc_page() at > all. Please, move up_read() outside khugepaged_alloc_page(). > I might be wrong. If we up_read in khugepaged_scan_pmd(), then if we round again do the for loop to get the next vma and handle it. Does we do this without holding the mmap_sem in any mode? And if the loop end, we have another up_read in breakouterloop. What if we have released the mmap_sem in collapse_huge_page()? -- Thanks. Zhang Yanfei -- 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/