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 S1033452AbdI0FEW (ORCPT <rfc822;w@1wt.eu>);
        Wed, 27 Sep 2017 01:04:22 -0400
Received: from LGEAMRELO13.lge.com ([156.147.23.53]:57602 "EHLO
        lgeamrelo13.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S935706AbdI0FEJ (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 27 Sep 2017 01:04:09 -0400
X-Original-SENDERIP: 156.147.1.126
X-Original-MAILFROM: minchan@kernel.org
X-Original-SENDERIP: 10.177.220.163
X-Original-MAILFROM: minchan@kernel.org
Date: Wed, 27 Sep 2017 14:04:01 +0900
From: Minchan Kim <minchan@kernel.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>,
        Rik van Riel <riel@redhat.com>, Shaohua Li <shli@kernel.org>,
        Hugh Dickins <hughd@google.com>, Fengguang Wu <fengguang.wu@intel.com>,
        Tim Chen <tim.c.chen@intel.com>, Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH] mm, swap: Make VMA based swap readahead configurable
Message-ID: <20170927050401.GA715@bbox>
References: <20170921013310.31348-1-ying.huang@intel.com>
 <20170926132129.dbtr2mof35x4j4og@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170926132129.dbtr2mof35x4j4og@dhcp22.suse.cz>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Sep 26, 2017 at 03:21:29PM +0200, Michal Hocko wrote:
> On Thu 21-09-17 09:33:10, Huang, Ying wrote:
> > From: Huang Ying <ying.huang@intel.com>
> > 
> > This patch adds a new Kconfig option VMA_SWAP_READAHEAD and wraps VMA
> > based swap readahead code inside #ifdef CONFIG_VMA_SWAP_READAHEAD/#endif.
> > This is more friendly for tiny kernels.
> 
> How (much)?
> 
> > And as pointed to by Minchan
> > Kim, give people who want to disable the swap readahead an opportunity
> > to notice the changes to the swap readahead algorithm and the
> > corresponding knobs.
> 
> Why would anyone want that?
> 
> Please note that adding new config options make the already complicated
> config space even more problematic so there should be a good reason to
> add one. Please make sure your justification is clear on why this is
> worth the future maintenance and configurability burden.

The problem is users have disabled swap readahead by echo 0 > /proc/sys/
vm/page-cluster are regressed by this new interface /sys/kernel/mm/swap/
vma_ra_max_order. Because for disabling readahead completely, they should
disable vma_ra_max_order as well as page-cluster from now on.

So, goal of new config to notice new feature to admins so they can be aware
of new konb vma_ra_max_order as well as page-cluster.
I canont think other better idea to preventing such regression.

http://lkml.kernel.org/r/%3C20170913014019.GB29422@bbox%3E