Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933643AbbHXJaE (ORCPT ); Mon, 24 Aug 2015 05:30:04 -0400 Received: from mail-wi0-f170.google.com ([209.85.212.170]:36189 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932793AbbHXJ3o (ORCPT ); Mon, 24 Aug 2015 05:29:44 -0400 Date: Mon, 24 Aug 2015 12:29:42 +0300 From: "Kirill A. Shutemov" To: Jesper Dangaard Brouer Cc: "Kirill A. Shutemov" , Andrew Morton , Hugh Dickins , Andrea Arcangeli , Dave Hansen , Vlastimil Babka , Johannes Weiner , Michal Hocko , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Lameter Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust Message-ID: <20150824092942.GA1994@node.dhcp.inet.fi> References: <1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com> <1439976106-137226-5-git-send-email-kirill.shutemov@linux.intel.com> <20150824015945.58b25f3a@brouer.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150824015945.58b25f3a@brouer.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3052 Lines: 86 On Mon, Aug 24, 2015 at 01:59:45AM +0200, Jesper Dangaard Brouer wrote: > On Wed, 19 Aug 2015 12:21:45 +0300 > "Kirill A. Shutemov" wrote: > > > Hugh has pointed that compound_head() call can be unsafe in some > > context. There's one example: > > > [...] > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 0735bc0a351a..a4c4b7d07473 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > [...] > > -/* > > - * If we access compound page synchronously such as access to > > - * allocated page, there is no need to handle tail flag race, so we can > > - * check tail flag directly without any synchronization primitive. > > - */ > > -static inline struct page *compound_head_fast(struct page *page) > > -{ > > - if (unlikely(PageTail(page))) > > - return page->first_page; > > - return page; > > -} > > - > [...] > > > @@ -548,13 +508,7 @@ static inline struct page *virt_to_head_page(const void *x) > > { > > struct page *page = virt_to_page(x); > > > > - /* > > - * We don't need to worry about synchronization of tail flag > > - * when we call virt_to_head_page() since it is only called for > > - * already allocated page and this page won't be freed until > > - * this virt_to_head_page() is finished. So use _fast variant. > > - */ > > - return compound_head_fast(page); > > + return compound_head(page); > > } > > I hope this does not slow down the SLAB/slub allocator? > (which calls virt_to_head_page() frequently) It should be slightly faster. Before: 00002e90 : 2e90: 8b 15 00 00 00 00 mov 0x0,%edx 2e96: 05 00 00 00 40 add $0x40000000,%eax 2e9b: c1 e8 0c shr $0xc,%eax 2e9e: c1 e0 05 shl $0x5,%eax 2ea1: 01 d0 add %edx,%eax 2ea3: 8b 10 mov (%eax),%edx 2ea5: f6 c6 80 test $0x80,%dh 2ea8: 75 06 jne 2eb0 2eaa: c3 ret 2eab: 90 nop 2eac: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi 2eb0: 8b 40 1c mov 0x1c(%eax),%eax 2eb3: c3 ret After: 00003070 : 3070: 8b 15 00 00 00 00 mov 0x0,%edx 3076: 05 00 00 00 40 add $0x40000000,%eax 307b: c1 e8 0c shr $0xc,%eax 307e: c1 e0 05 shl $0x5,%eax 3081: 01 d0 add %edx,%eax 3083: 8b 50 14 mov 0x14(%eax),%edx 3086: 8d 4a ff lea -0x1(%edx),%ecx 3089: f6 c2 01 test $0x1,%dl 308c: 0f 45 c1 cmovne %ecx,%eax 308f: c3 ret -- Kirill A. Shutemov -- 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/