Re: [PATCHv3 00/19] zram: convert to custom compression API and allow algorithms tuning

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/05/10 15:40), Herbert Xu wrote:
> > But in general case, a typical crypto API usage
> > 
> > 	tfm = crypto_alloc_comp(comp->name, 0, 0);
> > 
> > should become much more complex.  I'd say that, probably, developing
> > an entirely new sub-set of API would be simpler.
> 
> We could easily add a setparams interface for acomp to support
> this.  The form of parameters would be specific to each individual
> algorithm (but obviously all drivers for the same algorithm must
> use the same format).

For some algorithms params needs to be set before ctx is created.
For example zstd, crypto/zstd calls zstd_get_params(ZSTD_DEF_LEVEL, 0)
to estimate workspace size, which misses the opportunity to configure
it an way zram/zswap can benefit from, because those work with PAGE_SIZE
source buffer.  So for zram zstd_get_params(ZSTD_DEF_LEVEL, PAGE_SIZE)
is much better (it saves 1.2MB per ctx, which is per-CPU in zram).  Not
to mention that zstd_get_params(param->level, 0) is what we need at the
end.

And then drivers need to be re-implemented to support params.  For
example, crypto/lz4 should call LZ4_compress_fast() instead of
LZ4_compress_default(), because fact() accepts compression level,
which is a tunable value.