From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx104.postini.com [74.125.245.104]) by kanga.kvack.org (Postfix) with SMTP id 0F70D6B0072 for ; Thu, 25 Oct 2012 17:27:47 -0400 (EDT) Received: by mail-ie0-f169.google.com with SMTP id 10so3668133ied.14 for ; Thu, 25 Oct 2012 14:27:46 -0700 (PDT) Date: Thu, 25 Oct 2012 14:27:41 -0700 (PDT) From: Hugh Dickins Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2] In-Reply-To: <508912B0.7080805@gmail.com> Message-ID: References: <20121025023738.GA27001@redhat.com> <5088C51D.3060009@gmail.com> <508912B0.7080805@gmail.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Ni zhan Chen Cc: Dave Jones , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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. 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