linux-kernel - Re: [PATCH -mm -v4 3/5] mm, swap: VMA based swap readahead

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Message-Id: <20170914142140.9c5e3ef37e6bad1d68899e64@linux-foundation.org>
Date:   Thu, 14 Sep 2017 14:21:40 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     "Huang, Ying" <ying.huang@...el.com>, 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 -v4 3/5] mm, swap: VMA based swap readahead

On Thu, 14 Sep 2017 22:14:46 +0900 Minchan Kim <minchan@...nel.org> wrote:

> > Now.  Users can choose between VMA based readahead and original
> > readahead via a knob as follow at runtime,
> > 
> > /sys/kernel/mm/swap/vma_ra_enabled
> 
> It's not a config option and is enabled by default. IOW, it's under the radar
> so current users cannot notice it. That's why we want to emit big fat warnning.
> when old user set 0 to page-cluster. However, as Andrew said, it's lame.
> 
> If we make it config option, product maker/kernel upgrade user can have
> a chance to notice and read description so they could be aware of two weird
> knobs and help to solve the problem in advance without printk_once warn.
> If user has no interest about swap-readahead or skip the new config option
> by mistake, it works physcial readahead which means no regression.

Yup, a Kconfig option sounds like a good idea.  And that's a bit more
friendly to tiny kernels as well.