From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id 764788D0039 for ; Sun, 6 Mar 2011 14:39:11 -0500 (EST) Received: from hpaq11.eem.corp.google.com (hpaq11.eem.corp.google.com [172.25.149.11]) by smtp-out.google.com with ESMTP id p26Jd8DW031017 for ; Sun, 6 Mar 2011 11:39:09 -0800 Received: from gyb13 (gyb13.prod.google.com [10.243.49.77]) by hpaq11.eem.corp.google.com with ESMTP id p26Jd57U012611 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for ; Sun, 6 Mar 2011 11:39:07 -0800 Received: by gyb13 with SMTP id 13so1850377gyb.0 for ; Sun, 06 Mar 2011 11:39:05 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <4D6CA852.3060303@cn.fujitsu.com> Date: Sun, 6 Mar 2011 11:39:04 -0800 Message-ID: Subject: Re: [PATCH 2/4] slub,rcu: don't assume the size of struct rcu_head From: Hugh Dickins Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Pekka Enberg , Lai Jiangshan , Ingo Molnar , "Paul E. McKenney" , Eric Dumazet , "David S. Miller" , Matt Mackall , linux-mm@kvack.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org On Wed, Mar 2, 2011 at 4:32 AM, Christoph Lameter wrote: > On Tue, 1 Mar 2011, Hugh Dickins wrote: > >> > Struct page may be larger for debugging purposes already because of th= e >> > need for extended spinlock data. >> >> That was so for a long time, but I stopped it just over a year ago >> with commit a70caa8ba48f21f46d3b4e71b6b8d14080bbd57a, stop ptlock >> enlarging struct page. > > Strange. I just played around with in in January and the page struct size > changes when I build kernels with full debugging. I have some > cmpxchg_double patches here that depend on certain alignment in the page > struct. Debugging causes all that stuff to get out of whack so that I had > to do some special patches to make sure fields following the spinlock are > properly aligned when the sizes change. That puzzles me, it's not my experience and I don't have an explanation: do you have time to investigate? Uh oh, you're going to tell me you're working on an out-of-tree architecture with a million cpus ;) In that case, yes, I'm afraid I'll have to update the SPLIT_PTLOCK_CPUS defaulting (for a million - 1 even). > >> If a union leads to "random junk" overwriting the page->mapping field >> when the page is reused, and that junk could resemble the pointer in >> question, then KSM would mistakenly think it still owned the page. >> Very remote chance, and maybe it amounts to no more than a leak. =C2=A0B= ut >> I'd still prefer we keep page->mapping for pointers (sometimes with >> lower bits set as flags). > > DESTROY BY RCU uses the lru field which follows the mapping field in page > struct. Why would random junk overwrite the mapping field? Random junk does not overwrite the mapping field with the current implementation of DESTROY_BY_RCU. But you and Jiangshan were discussing how to change it, so I was warning of this issue with page->mapping. But I would anyway agree with Jiangshan, that it's preferable not to bloat struct page size just for this DESTROY_BY_RCU issue, even if it is only an issue when debugging. Hugh -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org