From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx173.postini.com [74.125.245.173]) by kanga.kvack.org (Postfix) with SMTP id CF9146B005A for ; Fri, 19 Oct 2012 11:53:47 -0400 (EDT) Message-ID: <5081777A.8050104@redhat.com> Date: Fri, 19 Oct 2012 11:53:30 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: question on NUMA page migration Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Andrea Arcangeli , Ingo Molnar , Linux Memory Management List , Mel Gorman , Linux kernel Mailing List 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? 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? :) -- All rights reversed -- 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