From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752795AbcDDFyP (ORCPT <rfc822;w@1wt.eu>);
	Mon, 4 Apr 2016 01:54:15 -0400
Received: from mail-pf0-f181.google.com ([209.85.192.181]:33159 "EHLO
	mail-pf0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750912AbcDDFyN (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 4 Apr 2016 01:54:13 -0400
Subject: Re: [PATCH v3 01/16] mm: use put_page to free page instead of
 putback_lru_page
To: Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
References: <1459321935-3655-1-git-send-email-minchan@kernel.org>
 <1459321935-3655-2-git-send-email-minchan@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, jlayton@poochiereds.net,
        bfields@fieldses.org, Vlastimil Babka <vbabka@suse.cz>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, koct9i@gmail.com,
        aquini@redhat.com, virtualization@lists.linux-foundation.org,
        Mel Gorman <mgorman@suse.de>, Hugh Dickins <hughd@google.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Rik van Riel <riel@redhat.com>, rknize@motorola.com,
        Gioh Kim <gi-oh.kim@profitbricks.com>,
        Sangseok Lee <sangseok.lee@lge.com>,
        Chan Gyun Jeong <chan.jeong@lge.com>,
        Al Viro <viro@ZenIV.linux.org.uk>, YiPing Xu <xuyiping@hisilicon.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
From: Balbir Singh <bsingharora@gmail.com>
Message-ID: <57020177.60006@gmail.com>
Date: Mon, 4 Apr 2016 15:53:59 +1000
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.7.1
MIME-Version: 1.0
In-Reply-To: <1459321935-3655-2-git-send-email-minchan@kernel.org>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org



On 30/03/16 18:12, 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. 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.
So effectively when we return from unmap_and_move() the page is either
put_page or putback_lru_page() and the page is gone from under us.