Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751991Ab2JZBsa (ORCPT ); Thu, 25 Oct 2012 21:48:30 -0400 Received: from mail-pb0-f46.google.com ([209.85.160.46]:36039 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750712Ab2JZBs3 (ORCPT ); Thu, 25 Oct 2012 21:48:29 -0400 Message-ID: <5089EBE1.1050009@gmail.com> Date: Fri, 26 Oct 2012 09:48:17 +0800 From: Ni zhan Chen User-Agent: Mozilla/5.0 (X11; Linux i686; rv:16.0) Gecko/20121011 Thunderbird/16.0.1 MIME-Version: 1.0 To: Hugh Dickins CC: Dave Jones , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2] References: <20121025023738.GA27001@redhat.com> <5088C51D.3060009@gmail.com> <508912B0.7080805@gmail.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2214 Lines: 55 On 10/26/2012 05:27 AM, Hugh Dickins wrote: > On Thu, 25 Oct 2012, Ni zhan Chen wrote: >> On 10/25/2012 02:59 PM, Hugh Dickins wrote: >>> On Thu, 25 Oct 2012, Ni zhan Chen wrote: >>>> I think it maybe caused by your commit [d189922862e03ce: shmem: fix >>>> negative >>>> rss in memcg memory.stat], one question: >>> Well, yes, I added the VM_BUG_ON in that commit. >>> >>>> if function shmem_confirm_swap confirm the entry has already brought back >>>> from swap by a racing thread, >>> The reverse: true confirms that the swap entry has not been brought back >>> from swap by a racing thread; false indicates that there has been a race. >>> >>>> then why call shmem_add_to_page_cache to add >>>> page from swapcache to pagecache again? >>> Adding it to pagecache again, after such a race, would set error to >>> -EEXIST (originating from radix_tree_insert); but we don't do that, >>> we add it to pagecache when it has not already been added. >>> >>> Or that's the intention: but Dave seems to have found an unexpected >>> exception, despite us holding the page lock across all this. >>> >>> (But if it weren't for the memcg and replace_page issues, I'd much >>> prefer to let shmem_add_to_page_cache discover the race as before.) >>> >>> Hugh >> Hi Hugh >> >> Thanks for your response. You mean the -EEXIST originating from >> radix_tree_insert, in radix_tree_insert: >> if (slot != NULL) >> return -EEXIST; >> But why slot should be NULL? if no race, the pagecache related radix tree >> entry should be RADIX_TREE_EXCEPTIONAL_ENTRY+swap_entry_t.val, where I miss? > I was describing what would happen in a case that should not exist, > that you had thought the common case. In actuality, the entry should > not be NULL, it should be as you say there. Thanks for your patience. So in the common case, the entry should be the value I mentioned, then why has this check? if (slot != NULL) return -EEXIST; the common case will return -EEXIST. > > Hugh > -- 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/