linux-kernel - Re: [PATCH 01/11] btrfs: add macros for compression type and level

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Date:   Thu, 31 Jan 2019 11:17:24 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     David Sterba <dsterba@...e.cz>
Cc:     Dennis Zhou <dennis@...nel.org>, David Sterba <dsterba@...e.com>,
        Josef Bacik <josef@...icpanda.com>, Chris Mason <clm@...com>,
        Omar Sandoval <osandov@...ndov.com>,
        Nick Terrell <terrelln@...com>, kernel-team@...com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/11] btrfs: add macros for compression type and level

On Thu, Jan 31, 2019 at 05:00:10PM +0100, David Sterba wrote:
> On Mon, Jan 28, 2019 at 04:24:27PM -0500, Dennis Zhou wrote:
> > It is very easy to miss places that rely on a certain bitshifting for
> > decyphering the type_level overloading. Make macros handle this instead.
> 
> This encoding was a simple way to introduce the combined type and level
> for zlib and it could certainly be improved. Either we'll use helpers
> like you suggest or add a new structure that contains type and level as
> members. That way we'd see where the level still matters and where the
> just the type is ok.
> 
> I haven't checked how much intrusive this would be, so that's for later
> consideration. Some indirection can be removed for the .set_level
> callbacks as it does not necessarily need to be a function so I'm
> expecting that the code around that would be touched anyway.

The only user of type_level is btrfs_compress_pages(). I do make an
extra call to .set_level() there just to be safe, but that can be taken
out as it should be correctly set from when we mounted.

I envisioned .set_level() to be called when setting the compression
level of the mount. This can be used in the same place if we were to add
per-file compression levels too. I mention later, but an important part
of .set_level() is to repurpose the meaning of 0 from meaning use the
default to meaning use any workspace available.

Thanks,
Dennis