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

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Message-ID: <20170927141723.bixcum3fler7q4w5@dhcp22.suse.cz>
Date:   Wed, 27 Sep 2017 16:17:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...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 Wed 27-09-17 23:10:08, Minchan Kim wrote:
> On Wed, Sep 27, 2017 at 03:50:34PM +0200, Michal Hocko wrote:
> > On Wed 27-09-17 22:41:17, Minchan Kim wrote:
> > > On Wed, Sep 27, 2017 at 03:22:41PM +0200, Michal Hocko wrote:
> > [...]
> > > > simply cannot disable swap readahead when page-cluster is 0?
> > > 
> > > That's was what I want really but Huang want to use two readahead
> > > algorithms in parallel so he wanted to keep two separated disable
> > > knobs.
> > 
> > If it breaks existing and documented behavior then it is a clear
> > regression and it should be fixed. I do not see why this should be
> > disputable at all.
> 
> Indeed but Huang doesn't think so. He has thought it's not a regression.
> Frankly speaking, I'm really bored of discussing with it.
> https://marc.info/?l=linux-mm&m=150526413319763&w=2

Then send a patch explaining why you consider this a regression with
some numbers backing it and I will happily ack it.

> So I passed the decision to Andrew.
> http://lkml.kernel.org/r/<20170913014019.GB29422@...x>
> 
> The config option idea is compromise approach although I don't like it
> and still believe it's simple clear *regression* so 0 page-cluster
> should keep the swap readahead disabled.

It is not a compromise. The regression is still there for many users
potentially (just consider zram distribution kernel users...).
-- 
Michal Hocko
SUSE Labs