Re: btrfs development - question about crypto api integration

[David Sterba <dsterba@suse.cz>]

On Fri, Nov 30, 2018 at 06:27:58PM +0200, Nikolay Borisov wrote:
> 
> 
> On 30.11.18 г. 17:22 ч., Chris Mason wrote:
> > On 29 Nov 2018, at 12:37, Nikolay Borisov wrote:
> > 
> >> On 29.11.18 г. 18:43 ч., Jean Fobe wrote:
> >>> Hi all,
> >>>     I've been studying LZ4 and other compression algorithms on the
> >>> kernel, and seen other projects such as zram and ubifs using the
> >>> crypto api. Is there a technical reason for not using the crypto api
> >>> for compression (and possibly for encryption) in btrfs?
> >>>     I did not find any design/technical implementation choices in
> >>> btrfs development in the developer's FAQ on the wiki. If I completely
> >>> missed it, could someone point me in the right direction?
> >>>     Lastly, if there is no technical reason for this, would it be
> >>> something interesting to have implemented?
> >>
> >> I personally think it would be better if btrfs' exploited the generic
> >> framework. And in fact when you look at zstd, btrfs does use the
> >> generic, low-level ZSTD routines but not the crypto library wrappers. 
> >> If
> >> I were I'd try and convert zstd (since it's the most recently added
> >> algorithm) to using the crypto layer to see if there are any lurking
> >> problems.
> > 
> > Back when I first added the zlib support, the zlib API was both easier 
> > to use and a better fit for our async worker threads.  That doesn't mean 
> > we shouldn't switch, it's just how we got to step one ;)
> 
> And what about zstd? WHy is it also using the low level api and not the
> crypto wrappers?

I think beacuse it just copied the way things already were.

Here's fs/ubifs/compress.c as an example use of the API in a filesystem:

ubifs_compress
  crypto_comp_compress
    crypto_comp_crt -- address of, dereference
    ->cot_commpress -- takes another address of something, indirect
		       function call
      with value 'crypto_compress'
	-- this does 2 pointer dereferences and indirect call to
	   coa_compress
	   coa_compress = lzo_compress
	     lzo_compress -- pointer dereferences for crypto context
	       lzo1x_1_compress -- through static __lzo_compress (no overhead)

while in btrfs:

btrfs_compress_pages
  ->compress_pages
    lzo1x_1_compress

The crypto API adds several pointer and function call indirectinos, I'm
not sure I want to get rid of the direct calls just yet.