linux-kernel - Re: [PATCH 4/5] mm:swap: respect page

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Date:   Tue, 12 Sep 2017 17:22:53 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team <kernel-team@....com>,
        Ilya Dryomov <idryomov@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 4/5] mm:swap: respect page_cluster for readahead

On Tue, Sep 12, 2017 at 04:07:01PM +0800, Huang, Ying wrote:
< snip >
> >> > My concern is users have been disabled swap readahead by page-cluster would
> >> > be regressed. Please take care of them.
> >> 
> >> How about disable VMA based swap readahead if zram used as swap?  Like
> >> we have done for hard disk?
> >
> > It could be with SWP_SYNCHRONOUS_IO flag which indicates super-fast,
> > no seek cost swap devices if this patchset is merged so VM automatically
> > disables readahead. It is in my TODO but it's orthogonal work.
> >
> > The problem I raised is "Why shouldn't we obey user's decision?",
> > not zram sepcific issue.
> >
> > A user has used SSD as swap devices decided to disable swap readahead
> > by some reason(e.g., small memory system). Anyway, it has worked
> > via page-cluster for a several years but with vma-based swap devices,
> > it doesn't work any more.
> 
> Can they add one more line to their configuration scripts?
> 
> echo 0 > /sys/kernel/mm/swap/vma_ra_max_order

We call it as "regression", don't we?