From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f170.google.com (mail-wi0-f170.google.com [209.85.212.170]) by kanga.kvack.org (Postfix) with ESMTP id 34C096B0038 for ; Tue, 18 Aug 2015 14:24:46 -0400 (EDT) Received: by wicne3 with SMTP id ne3so102075695wic.0 for ; Tue, 18 Aug 2015 11:24:45 -0700 (PDT) Received: from mail-wi0-f176.google.com (mail-wi0-f176.google.com. [209.85.212.176]) by mx.google.com with ESMTPS id s1si28673076wif.85.2015.08.18.11.24.44 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 18 Aug 2015 11:24:44 -0700 (PDT) Received: by wibhh20 with SMTP id hh20so116176951wib.0 for ; Tue, 18 Aug 2015 11:24:43 -0700 (PDT) Date: Tue, 18 Aug 2015 21:24:41 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCHv2 4/4] mm: make compound_head() robust Message-ID: <20150818182441.GC21383@node.dhcp.inet.fi> References: <1439824145-25397-1-git-send-email-kirill.shutemov@linux.intel.com> <1439824145-25397-5-git-send-email-kirill.shutemov@linux.intel.com> <20150818112022.GJ5033@dhcp22.suse.cz> <20150818164112.GN5033@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150818164112.GN5033@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: "Kirill A. Shutemov" , Andrew Morton , Hugh Dickins , Andrea Arcangeli , Dave Hansen , Vlastimil Babka , Johannes Weiner , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Tue, Aug 18, 2015 at 06:41:13PM +0200, Michal Hocko wrote: > On Tue 18-08-15 13:20:22, Michal Hocko wrote: > > On Mon 17-08-15 18:09:05, Kirill A. Shutemov wrote: > > > Hugh has pointed that compound_head() call can be unsafe in some > > > context. There's one example: > > > > > > CPU0 CPU1 > > > > > > isolate_migratepages_block() > > > page_count() > > > compound_head() > > > !!PageTail() == true > > > put_page() > > > tail->first_page = NULL > > > head = tail->first_page > > > alloc_pages(__GFP_COMP) > > > prep_compound_page() > > > tail->first_page = head > > > __SetPageTail(p); > > > !!PageTail() == true > > > > > > > > > The race is pure theoretical. I don't it's possible to trigger it in > > > practice. But who knows. > > > > > > We can fix the race by changing how encode PageTail() and compound_head() > > > within struct page to be able to update them in one shot. > > > > > > The patch introduces page->compound_head into third double word block in > > > front of compound_dtor and compound_order. That means it shares storage > > > space with: > > > > > > - page->lru.next; > > > - page->next; > > > - page->rcu_head.next; > > > - page->pmd_huge_pte; > > > > > > That's too long list to be absolutely sure, but looks like nobody uses > > > bit 0 of the word. It can be used to encode PageTail(). And if the bit > > > set, rest of the word is pointer to head page. > > > > I didn't look too closely but the general idea makes sense to me and the > > overal code simplification is sound. I will give it more detailed review > > after I sort out other stuff. > > AFICS page::first_page wasn't used outside of compound page logic so you > should remove it in this patch. The rest looks good to me. I missed it by accident during rework for v2. Will fix. > Acked-by: Michal Hocko Thanks! -- Kirill A. Shutemov -- 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