Re: [RFC PATCH 6/6] btrfs: zlib: add support for zlib-deflate through acomp

[Josef Bacik <josef@toxicpanda.com>]

On Fri, Apr 26, 2024 at 11:54:29AM +0100, Giovanni Cabiddu wrote:
> From: Weigang Li <weigang.li@intel.com>
> 
> Add support for zlib compression and decompression through the acomp
> APIs.
> Input pages are added to an sg-list and sent to acomp in one request.
> Since acomp is asynchronous, the thread is put to sleep and then the CPU
> is freed up. Once compression is done, the acomp callback is triggered
> and the thread is woke up.
> 
> This patch doesn't change the BTRFS disk format, this means that files
> compressed by hardware engines can be de-compressed by the zlib software
> library, and vice versa.
> 
> Limitations:
>   * The implementation tries always to use an acomp even if only
>     zlib-deflate-scomp is present
>   * Acomp does not provide a way to support compression levels

That's a non-starter.  We can't just lie to the user about the compression level
that is being used.  If the user just does "-o compress=zlib" then you need to
update btrfs_compress_set_level() to figure out the compression level that acomp
is going to use and set that appropriately, so we can report to the user what is
actually being used.

Additionally if a user specifies a compression level you need to make sure we
don't do acomp if it doesn't match what acomp is going to do.

Finally, for the normal code review, there's a bunch of things that need to be
fixed up before I take a closer look

- We don't use pr_(), we have btrfs specific printk helpers, please use those.
- We do 1 variable per line, fix up the variable declarations in your functions.

Thanks,

Josef