linux-kernel - Re: [PATCH] mm/zswap: change params from hidden to ro

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Message-ID: <20131122073851.GB1853@hp530>
Date:	Fri, 22 Nov 2013 08:38:54 +0100
From:	Vladimir Murzin <murzin.v@...il.com>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	linux-mm@...ck.org, Seth Jennings <sjennings@...iantweb.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Bob Liu <bob.liu@...cle.com>, Minchan Kim <minchan@...nel.org>,
	Weijie Yang <weijie.yang@...sung.com>
Subject: Re: [PATCH] mm/zswap: change params from hidden to ro

On Wed, Nov 20, 2013 at 12:52:47PM -0500, Dan Streetman wrote: > On Wed, Nov
20, 2013 at 12:33 PM, Vladimir Murzin <murzin.v@...il.com> wrote: > > Hi Dan!
> >
> > On Wed, Nov 20, 2013 at 11:38:42AM -0500, Dan Streetman wrote:
> >> The "compressor" and "enabled" params are currently hidden,
> >> this changes them to read-only, so userspace can tell if
> >> zswap is enabled or not and see what compressor is in use.
> >
> > Could you elaborate more why this pice of information is necessary for
> > userspace?
> 
> For anyone interested in zswap, it's handy to be able to tell if it's
> enabled or not ;-)  Technically people can check to see if the zswap
> debug files are in /sys/kernel/debug/zswap, but I think the actual
> "enabled" param is more obvious.  And the compressor param is really
> the only way anyone from userspace can see what compressor's being
> used; that's helpful to know for anyone that might want to be using a
> non-default compressor.

So, it is needed for user not userspace? I tend to think that users are smart
enough to check cmdline for that. 

AFAICS module_param exist here to provide simplest ability to handle the
setting via cmdline. zsawap is not able to be loaded as a module for now. If
it could, than there was reason to check which params were used while module
loading and guess how they affected zswap state. 

> 
> And of course, eventually we'll want to make the params writable, so
> the compressor can be changed dynamically, and zswap can be enabled or
> disabled dynamically (or at least enabled after boot).

module_params is not the best place to handle this, because they do not
provide any hooks for handling write unless I missed something. However,
making zswap state dynamically adjustable require not only setting the
numbers, but handling the correctness switching from one state to another.

Vladimir
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