linux-kernel - Re: [PATCH] mm, swap: Make VMA based swap readahead configurable

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Date:   Wed, 27 Sep 2017 14:04:01 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...hat.com>, Shaohua Li <shli@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH] mm, swap: Make VMA based swap readahead configurable

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@...el.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