Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753117AbbHUVPj (ORCPT ); Fri, 21 Aug 2015 17:15:39 -0400 Received: from resqmta-po-06v.sys.comcast.net ([96.114.154.165]:55118 "EHLO resqmta-po-06v.sys.comcast.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752556AbbHUVPi (ORCPT ); Fri, 21 Aug 2015 17:15:38 -0400 Date: Fri, 21 Aug 2015 16:15:35 -0500 (CDT) From: Christoph Lameter X-X-Sender: cl@east.gentwo.org To: Andrew Morton cc: "Kirill A. Shutemov" , "Kirill A. Shutemov" , Hugh Dickins , Andrea Arcangeli , Dave Hansen , Vlastimil Babka , Johannes Weiner , Michal Hocko , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust In-Reply-To: <20150821123458.b3a6947135d5b506a34abc61@linux-foundation.org> Message-ID: References: <1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com> <1439976106-137226-5-git-send-email-kirill.shutemov@linux.intel.com> <20150820163643.dd87de0c1a73cb63866b2914@linux-foundation.org> <20150821121028.GB12016@node.dhcp.inet.fi> <20150821193109.GA14785@node.dhcp.inet.fi> <20150821123458.b3a6947135d5b506a34abc61@linux-foundation.org> Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1398 Lines: 34 On Fri, 21 Aug 2015, Andrew Morton wrote: > On Fri, 21 Aug 2015 22:31:09 +0300 "Kirill A. Shutemov" wrote: > > > On Fri, Aug 21, 2015 at 11:11:27AM -0500, Christoph Lameter wrote: > > > On Fri, 21 Aug 2015, Kirill A. Shutemov wrote: > > > > > > > > Is this really true? For example if it's a slab page, will that page > > > > > ever be inspected by code which is looking for the PageTail bit? > > > > > > > > +Christoph. > > > > > > > > What we know for sure is that space is not used in tail pages, otherwise > > > > it would collide with current compound_dtor. > > > > > > Sl*b allocators only do a virt_to_head_page on tail pages. > > > > The question was whether it's safe to assume that the bit 0 is always zero > > in the word as this bit will encode PageTail(). > > That wasn't my question actually... > > What I'm wondering is: if this page is being used for slab, will any > code path ever run PageTail() against it? If not, we don't need to be > concerned about that bit. virt_to_head_page will run PageTail because it uses compound_head(). And compound_head needs to use the first_page pointer if its a tail page. -- 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/