From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx126.postini.com [74.125.245.126]) by kanga.kvack.org (Postfix) with SMTP id 5B1D56B0068 for ; Thu, 3 Jan 2013 00:10:55 -0500 (EST) Received: by mail-pa0-f42.google.com with SMTP id rl6so8443094pac.29 for ; Wed, 02 Jan 2013 21:10:54 -0800 (PST) Date: Wed, 2 Jan 2013 21:10:48 -0800 (PST) From: Hugh Dickins Subject: Re: [PATCH v7 1/2] KSM: numa awareness sysfs knob In-Reply-To: <1357030004.1379.4.camel@kernel.cn.ibm.com> Message-ID: References: <20121224050817.GA25749@kroah.com> <1356658337-12540-1-git-send-email-pholasek@redhat.com> <1357030004.1379.4.camel@kernel.cn.ibm.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Simon Jeons 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 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 :) > > 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. 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 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 -- 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