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

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (05/27/16 13:43), Minchan Kim wrote:
[..]
> >  modprobe zram
> >  cat /proc/crypto | grep -i lz4
> >  modprobe lz4
> >  cat /proc/crypto | grep -i lz4
> > name         : lz4
> > driver       : lz4-generic
> > module       : lz4
> > 
> > So the user can't tell exactly if the lz4 is really supported
> > from /proc/crypto output, unless someone or something has loaded
> > it.
> > 
> > This patch also adds crypto_has_comp() to zcomp_available_show().
> 
> crypto_has_comp works regardless of that whether module is loading or not?
> IOW, currently, if lz4 modules is not loading, but crypto_has_comp return
> true about lz4 module.
> Right?

correct. crypto_has_comp() regardless the module being loaded.

# modprobe zram
# cat /proc/crypto | egrep -e "lzo|lz4|deflate"

# echo lzo > /sys/block/zram0/comp_algorithm 
# cat /proc/crypto | egrep -e "lzo|lz4|deflate"
name         : lzo
driver       : lzo-generic
module       : lzo

# echo lz4 > /sys/block/zram0/comp_algorithm 
# cat /proc/crypto | egrep -e "lzo|lz4|deflate"
name         : lz4
driver       : lz4-generic
module       : lz4
name         : lzo
driver       : lzo-generic
module       : lzo

# echo deflate > /sys/block/zram0/comp_algorithm 
# cat /proc/crypto | egrep -e "lzo|lz4|deflate"
name         : deflate
driver       : deflate-generic
module       : deflate
name         : lz4
driver       : lz4-generic
module       : lz4
name         : lzo
driver       : lzo-generic
module       : lzo


what is does, tho, it modprobs() the module upon the first
request:

crypto_has_comp(...)
	crypto_has_alg()
		crypto_alg_mod_lookup()
			crypto_larval_lookup()
				request_module("crypto-%s", name)
					__request_module()
						call_modprobe()

and this is when /proc/crypto is getting updated. otherwise user has
no information (well, unless modules were loaded by something/someome
else, or crypto compressors were built-in into the kernel). I'm not
aware of any other way to achieve this functionality for zram.


> > We store all the compression algorithms names in zcomp's `backends'
> > array, regardless the CONFIG_CRYPTO_FOO configuration, but show
> 
> Then, you mean we should add new string into backend array whenever
> adding new crypto compatible compression algorithm?

yes. which looks quite trivial: adding or removing a string to/from
the array.


> >  cat /proc/crypto | grep -i lz4
> > 
> >  cat /sys/block/zram0/comp_algorithm
> > [lzo] lz4 deflate lz4hc 842
> 
> So, when lzo module is loading?

when we execute crypto_has_comp("lzo") for the first time.


that's why doing just

# modprobe zram

will not cause /proc/crypto update


# modprobe zram
# cat /proc/crypto | egrep -e "lzo|lz4|deflate"


crypto_has_comp() updates it -- when we read or write
from/to comp_algorithm:

# cat /sys/block/zram0/comp_algorithm
[lzo] lz4 deflate lz4hc 842

# cat /proc/crypto | egrep -e "lzo|lz4|deflate"
name         : lzo
driver       : lzo-generic
module       : lzo


but I didn't want zram to depend on this, or to depend on
/proc/crypto content; that's why I did it the way it is.

	-ss