linux-kernel - Re: [PATCH v2] zram: introduce comp algorithm fallback functionality

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Date:	Thu, 10 Sep 2015 14:33:19 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Luis Henriques <luis.henriques@...onical.com>,
	Nitin Gupta <ngupta@...are.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zram: introduce comp algorithm fallback functionality

Hello,

On (09/10/15 14:03), Minchan Kim wrote:
[..]
>
> I guess most of scripts have checked result of his doing so if we
> removes it, it will break them.

to be honest, we never documented or required any of those. the only source
of information for the user space -- zram.txt documentation -- simply says
to do 'echo 3 > /sys/block/zram0/max_comp_streams' or any other bunch of
'echo'-s. so, technically, a user is free to simply copy-paste what we do
in zram.txt to his zram.sh and call it a "recommended way of doing things
in zram".

besides, zram.txt is outdated. for example there is no mem_used_max
documentation.

we need to do better job documenting. I'll try to take a look on this later
this week.

> Given that, every success of "echo zzz > /sys/block/zram0/comp_algorithm"
> makes users to be confused that he might think to success to change algorithm
> in runtime. IOW, it should return error which is more intuitive forme.

well, not quite like that. we return -EINVAL on invalid output since
d93435c3fba4a47b773693b0c8992470d38510d5. this patch does not change
anything from this pov. it does, however, change the behaviour of
disksize store that follows.

I'm fine when the motivation for this patch is to introduce the
"fallback" mechanism (like zswap fallbacks to default compressor, f.e.),
but it wasn't the case -- I rewrote the patch slightly, reworded the
commit message and put some reasoning to this patch.

	-ss
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