From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f180.google.com (mail-wi0-f180.google.com [209.85.212.180]) by kanga.kvack.org (Postfix) with ESMTP id 3434C6B0038 for ; Tue, 9 Jun 2015 12:14:30 -0400 (EDT) Received: by wiwd19 with SMTP id d19so23784759wiw.0 for ; Tue, 09 Jun 2015 09:14:29 -0700 (PDT) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id y11si3934132wiv.114.2015.06.09.09.14.27 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 09 Jun 2015 09:14:28 -0700 (PDT) Message-ID: <557710E1.6060103@suse.cz> Date: Tue, 09 Jun 2015 18:14:25 +0200 From: Vlastimil Babka MIME-Version: 1.0 Subject: Re: [PATCH V3] mm:add VM_BUG_ON_PAGE() for page_mapcount() References: <010b01d012ca$05244060$0f6cc120$@alibaba-inc.com> <35FD53F367049845BC99AC72306C23D103E688B313F9@CNBJMBX05.corpusers.net> <35FD53F367049845BC99AC72306C23D103E688B313FA@CNBJMBX05.corpusers.net> In-Reply-To: <35FD53F367049845BC99AC72306C23D103E688B313FA@CNBJMBX05.corpusers.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Wang, Yalin" , 'Hillf Danton' , 'linux-kernel' , "'linux-mm@kvack.org'" , "'linux-arm-kernel@lists.infradead.org'" , 'Andrew Morton' Cc: "Kirill A. Shutemov" , David Rientjes On 12/08/2014 10:59 AM, Wang, Yalin wrote: > This patch add VM_BUG_ON_PAGE() for slab page, > because _mapcount is an union with slab struct in struct page, > avoid access _mapcount if this page is a slab page. > Also remove the unneeded bracket. > > Signed-off-by: Yalin Wang > --- > include/linux/mm.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index b464611..a117527 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -449,7 +449,8 @@ static inline void page_mapcount_reset(struct page *page) > > static inline int page_mapcount(struct page *page) > { > - return atomic_read(&(page)->_mapcount) + 1; > + VM_BUG_ON_PAGE(PageSlab(page), page); > + return atomic_read(&page->_mapcount) + 1; > } > I think this might theoretically trigger on the following code in compaction's isolate_migratepages_block(): /* * Migration will fail if an anonymous page is pinned in memory, * so avoid taking lru_lock and isolating it unnecessarily in an * admittedly racy check. */ if (!page_mapping(page) && page_count(page) > page_mapcount(page)) continue; This is done after PageLRU() was positive, but the lru_lock might be not taken yet. So, there's some time window during which the page might have been reclaimed from LRU and become a PageSlab(page). !page_mapping(page) will be true in that case so it will proceed with page_mapcount(page) test and trigger the VM_BUG_ON. (That test was added by DavidR year ago in commit 119d6d59dcc0980dcd581fdadb6b2033b512a473) Vlastimil > static inline int page_count(struct page *page) > -- 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