From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754131Ab3ADAYK (ORCPT ); Thu, 3 Jan 2013 19:24:10 -0500 Received: from mail-pa0-f47.google.com ([209.85.220.47]:50272 "EHLO mail-pa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753777Ab3ADAYH (ORCPT ); Thu, 3 Jan 2013 19:24:07 -0500 Message-ID: <1357259044.4930.4.camel@kernel.cn.ibm.com> Subject: Re: [PATCH v7 1/2] KSM: numa awareness sysfs knob From: Simon Jeons To: Hugh Dickins Cc: Petr Holasek , Andrea Arcangeli , Andrew Morton , Izik Eidus , Rik van Riel , David Rientjes , Sasha Levin , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Anton Arapov Date: Thu, 03 Jan 2013 18:24:04 -0600 In-Reply-To: References: <20121224050817.GA25749@kroah.com> <1356658337-12540-1-git-send-email-pholasek@redhat.com> <1357030004.1379.4.camel@kernel.cn.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4 (3.4.4-2.fc17) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2013-01-02 at 21:10 -0800, Hugh Dickins wrote: > On Tue, 1 Jan 2013, Simon Jeons wrote: > > > > Hi Petr and Hugh, > > > > One offline question, thanks for your clarify. > > Perhaps not as offline as you intended :) Hi Hugh, Thanks for your detail explanation. :) > > > > > How to understand age = (unsigned char)(ksm_scan.seqnr - > > rmap_item->address);? It used for what? > > As you can see, remove_rmap_item_from_tree uses it to decide whether > or not it should rb_erase the rmap_item from the unstable_tree. > > Every full scan of all the rmap_items, we increment ksm_scan.seqnr, > forget the old unstable_tree (it would just be a waste of processing > to remove every node one by one), and build up the unstable_tree afresh. > When the rmap_items left over from the previous scan will be removed? > That works fine until we need to remove an rmap_item: then we have to be > very sure to remove it from the unstable_tree if it's already been linked > there during this scan, but ignore its rblinkage if that's just left over > from the previous scan. > > A single bit would be enough to decide this; but we got it troublesomely > wrong in the early days of KSM (didn't always visit every rmap_item each > scan), so it's convenient to use 8 bits (the low unsigned char, stored When the scenario didn't always visit every rmap_item each scan can occur? > below the FLAGs and below the page-aligned address in the rmap_item - > there's lots of them, best keep them as small as we can) and do a > BUG_ON(age > 1) if we made a mistake. > > We haven't hit that BUG_ON in over three years: if we need some more > bits for something, we can cut the age down to one or two bits. > > Hugh