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

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Message-ID: <20150910055814.GA30419@bbox>
Date:	Thu, 10 Sep 2015 14:58:14 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Luis Henriques <luis.henriques@...onical.com>,
	Nitin Gupta <ngupta@...are.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zram: introduce comp algorithm fallback functionality

On Thu, Sep 10, 2015 at 02:33:19PM +0900, Sergey Senozhatsky wrote:
> 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".

Agree. That's why we spend a lot discussion for this small change.

> 
> besides, zram.txt is outdated.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.

Thanks.

> 
> 
> > 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 said like that you cut off.

"Although we could change ABI of optional parameters into no failure smoothly"
                                                          ^^^^^^^^^^

> 
> 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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