From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx196.postini.com [74.125.245.196]) by kanga.kvack.org (Postfix) with SMTP id 9DC246B0071 for ; Thu, 25 Oct 2012 21:48:29 -0400 (EDT) Received: by mail-da0-f41.google.com with SMTP id i14so1160662dad.14 for ; Thu, 25 Oct 2012 18:48:28 -0700 (PDT) Message-ID: <5089EBE1.1050009@gmail.com> Date: Fri, 26 Oct 2012 09:48:17 +0800 From: Ni zhan Chen MIME-Version: 1.0 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: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: Dave Jones , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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, 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