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From: "Huang\, Ying" <ying.huang@intel.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>,
        "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
References: <20170926132129.dbtr2mof35x4j4og@dhcp22.suse.cz>
        <20170927050401.GA715@bbox>
        <20170927074835.37m4dclmew5ecli2@dhcp22.suse.cz>
        <20170927080432.GA1160@bbox>
        <20170927083512.dydqlqezh5polggb@dhcp22.suse.cz>
        <20170927131511.GA338@bgram>
        <20170927132241.tshup6kcwe5pcxek@dhcp22.suse.cz>
        <20170927134117.GB338@bgram>
        <20170927135034.yatxlhvunawzmcar@dhcp22.suse.cz>
        <20170927141008.GA1278@bgram>
        <20170927141723.bixcum3fler7q4w5@dhcp22.suse.cz>
Date: Thu, 28 Sep 2017 09:02:20 +0800
In-Reply-To: <20170927141723.bixcum3fler7q4w5@dhcp22.suse.cz> (Michal Hocko's
        message of "Wed, 27 Sep 2017 16:17:23 +0200")
Message-ID: <87mv5f8wkj.fsf@yhuang-dev.intel.com>
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Hi, Michal,

Michal Hocko <mhocko@kernel.org> writes:

> 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.

I still think there may be a performance regression for some users
because of the change of the algorithm and the knobs, and the
performance regression can be resolved via setting the new knob.  But I
don't think there will be a functionality regression.  Do you agree?

Best Regards,
Huang, Ying

>> So I passed the decision to Andrew.
>> http://lkml.kernel.org/r/<20170913014019.GB29422@bbox>
>> 
>> 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...).