From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx138.postini.com [74.125.245.138]) by kanga.kvack.org (Postfix) with SMTP id 0E2C76B0062 for ; Sat, 20 Oct 2012 22:39:35 -0400 (EDT) Received: by mail-ie0-f169.google.com with SMTP id 10so2869217ied.14 for ; Sat, 20 Oct 2012 19:39:34 -0700 (PDT) Message-ID: <50836060.4050408@gmail.com> Date: Sun, 21 Oct 2012 10:39:28 +0800 From: Ni zhan Chen MIME-Version: 1.0 Subject: Re: question on NUMA page migration References: <5081777A.8050104@redhat.com> In-Reply-To: <5081777A.8050104@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Rik van Riel Cc: Peter Zijlstra , Andrea Arcangeli , Ingo Molnar , Linux Memory Management List , Mel Gorman , Linux kernel Mailing List On 10/19/2012 11:53 PM, Rik van Riel wrote: > Hi Andrea, Peter, > > I have a question on page refcounting in your NUMA > page migration code. > > In Peter's case, I wonder why you introduce a new > MIGRATE_FAULT migration mode. If the normal page > migration / compaction logic can do without taking > an extra reference count, why does your code need it? Hi Rik van Riel, This is which part of codes? Why I can't find MIGRATE_FAULT in latest v3.7-rc2? Regards, Chen > > In Andrea's case, we have a comment suggesting an > extra refcount is needed, immediately followed by > a put_page: > > /* > * Pin the head subpage at least until the first > * __isolate_lru_page succeeds (__isolate_lru_page pins it > * again when it succeeds). If we unpin before > * __isolate_lru_page successd, the page could be freed and > * reallocated out from under us. Thus our previous checks on > * the page, and the split_huge_page, would be worthless. > * > * We really only need to do this if "ret > 0" but it doesn't > * hurt to do it unconditionally as nobody can reference > * "page" anymore after this and so we can avoid an "if (ret > > * 0)" branch here. > */ > put_page(page); > > This also confuses me. > > If we do not need the extra refcount (and I do not > understand why NUMA migrate-on-fault needs one more > refcount than normal page migration), we can get > rid of the MIGRATE_FAULT mode. > > If we do need the extra refcount, why is normal > page migration safe? :) > -- 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