linux-kernel - Re: [PATCH 5/7] zram: use crypto api to check alg availability

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Date:	Mon, 30 May 2016 13:47:18 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/7] zram: use crypto api to check alg availability

Hi Sergey,

On Sun, May 29, 2016 at 12:24:58PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (05/27/16 18:04), Minchan Kim wrote:
> > > It is fundamentally impossible to get a list of all *potential*
> > > algorithms if you allow loadable modules.  By definition someone
> > > could load a new module and thus introduce a new algorithm.
> > 
> > Thanks for the quick response. I understand now.
> > 
> > Sergey,
> > 
> > Then, shouldn't we add functionality to load new algorithm although
> > there is no entry in zcomp backend array?
> 
> can do, sure: make find_backend() depend on crypto_has_comp() only,
> *may be* printk a message when we have a mismatch in backends array
> and crypto_has_comp(). but not really sure about the latter one.

Me, too. We don't need to warn about that. Instead, we should allow loading
other algorithm which doesn't live in backend array more clearly.
We need to update documentation to clear it out.

> 
> > The main reason we changed to crypto is to support various compression
> > algorithm for zram so we should be able to support anyone who want to
> > use custom crypto compression module.
> 
> yes.
> 
> > A idea is that let's not adding new compression string into backend
> > array from now on and deprecates /zram/comp_algorithm to show name.
> 
> I'd really prefer not to deprecate /zram/comp_algorithm now.
> 
> > So, from now on, users should be aware of the compression naming and
> > load module by oneself. It's not kind but I hope admins already knows
> > what cryto compressor they have.
> 
> um, I'm not in love with this approach.
> 
> what admins? zram users may be completely unaware of the existence of
> the crypto API and the kernel rebuild process. for some people switching
> to new zram will be a matter of "apt-get upgrade/pacman -Syu", not
> "git am ...; make menuconfig; make -j...". this looks like a major step
> back. keeping this dummy list of compressing backends around does not take
> a lot of effort, especially if we will make it unimportant for find_backend()
> and only use it for that nice user-friendly `cat /zram/comp_algorithm`
> output.

I am not against with it. Instead, please clear it out to write down docuement
"comp_algorithm is just optional so user can load any new algorithm like this
 blah blah"

Thanks.
> 
> 	-ss