Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755394AbcCNIsx (ORCPT ); Mon, 14 Mar 2016 04:48:53 -0400 Received: from mx2.suse.de ([195.135.220.15]:33864 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755287AbcCNIsk (ORCPT ); Mon, 14 Mar 2016 04:48:40 -0400 From: Vlastimil Babka Subject: Re: [PATCH v1 01/19] mm: use put_page to free page instead of putback_lru_page To: Minchan Kim , Andrew Morton References: <1457681423-26664-1-git-send-email-minchan@kernel.org> <1457681423-26664-2-git-send-email-minchan@kernel.org> Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, jlayton@poochiereds.net, bfields@fieldses.org, Joonsoo Kim , koct9i@gmail.com, aquini@redhat.com, virtualization@lists.linux-foundation.org, Mel Gorman , Hugh Dickins , Sergey Senozhatsky , rknize@motorola.com, Rik van Riel , Gioh Kim , Naoya Horiguchi Message-ID: <56E67AE1.60700@suse.cz> Date: Mon, 14 Mar 2016 09:48:33 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <1457681423-26664-2-git-send-email-minchan@kernel.org> Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1868 Lines: 45 On 03/11/2016 08:30 AM, Minchan Kim wrote: > Procedure of page migration is as follows: > > First of all, it should isolate a page from LRU and try to > migrate the page. If it is successful, it releases the page > for freeing. Otherwise, it should put the page back to LRU > list. > > For LRU pages, we have used putback_lru_page for both freeing > and putback to LRU list. It's okay because put_page is aware of > LRU list so if it releases last refcount of the page, it removes > the page from LRU list. However, It makes unnecessary operations > (e.g., lru_cache_add, pagevec and flags operations. Yeah, and compaction (perhaps also other migration users) has to drain the lru pvec... Getting rid of this stuff is worth even by itself. > It would be > not significant but no worth to do) and harder to support new > non-lru page migration because put_page isn't aware of non-lru > page's data structure. > > To solve the problem, we can add new hook in put_page with > PageMovable flags check but it can increase overhead in > hot path and needs new locking scheme to stabilize the flag check > with put_page. > > So, this patch cleans it up to divide two semantic(ie, put and putback). > If migration is successful, use put_page instead of putback_lru_page and > use putback_lru_page only on failure. That makes code more readable > and doesn't add overhead in put_page. I had an idea of checking for count==1 in putback_lru_page() which would take the put_page() shortcut from there. But maybe it can't be done nicely without races. > Cc: Vlastimil Babka > Cc: Mel Gorman > Cc: Hugh Dickins > Cc: Naoya Horiguchi > Signed-off-by: Minchan Kim Acked-by: Vlastimil Babka Note in -next/after 4.6-rc1 this will need some rebasing though.