From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f174.google.com (mail-ig0-f174.google.com [209.85.213.174]) by kanga.kvack.org (Postfix) with ESMTP id D8BAA6B0253 for ; Fri, 21 Aug 2015 17:15:38 -0400 (EDT) Received: by igcse8 with SMTP id se8so9627143igc.1 for ; Fri, 21 Aug 2015 14:15:38 -0700 (PDT) Received: from resqmta-po-10v.sys.comcast.net (resqmta-po-10v.sys.comcast.net. [2001:558:fe16:19:96:114:154:169]) by mx.google.com with ESMTPS id h191si2369473ioe.191.2015.08.21.14.15.37 for (version=TLSv1.2 cipher=RC4-SHA bits=128/128); Fri, 21 Aug 2015 14:15:38 -0700 (PDT) Date: Fri, 21 Aug 2015 16:15:35 -0500 (CDT) From: Christoph Lameter 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: owner-linux-mm@kvack.org List-ID: 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 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, 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