linux-kernel - Re: [PATCH] btrfs: Accept and ignore compression level for lzo

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Message-ID: <20250822184547.GX22430@suse.cz>
Date: Fri, 22 Aug 2025 20:45:47 +0200
From: David Sterba <dsterba@...e.cz>
To: Calvin Owens <calvin@...nvd.org>
Cc: Qu Wenruo <quwenruo.btrfs@....com>, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, Daniel Vacek <neelx@...e.com>,
	David Sterba <dsterba@...e.com>, Josef Bacik <josef@...icpanda.com>,
	Chris Mason <clm@...com>
Subject: Re: [PATCH] btrfs: Accept and ignore compression level for lzo

On Fri, Aug 22, 2025 at 02:28:29AM -0700, Calvin Owens wrote:
> On Friday 08/22 at 17:57 +0930, Qu Wenruo wrote:
> > 在 2025/8/22 17:15, Calvin Owens 写道:
> > > The compression level is meaningless for lzo, but before commit
> > > 3f093ccb95f30 ("btrfs: harden parsing of compression mount options"),
> > > it was silently ignored if passed.
> > 
> > Since LZO doesn't support compression level, why providing a level parameter
> > in the first place?
> 
> Interpreting "no level" as "level is always one" doesn't seem that
> unreasonable to me, especially since it has worked forever.

As it currently works, no level means use the default, which is defined
for all compression. For LZO it's implicit and 1.

> > I think it's time for those users to properly update their mount options.
> 
> It's a user visable regression, and fixing it has zero possible
> downside. I think you should take my patch :)

I tend to agree this is a usability regression, even if LZO is a bit odd
with levels, accepting the allowed values should work.

The mount options and level combinations that should work:

- compress=NAME   - use default level for NAME
- compress=NAME:0 - use default, while accepting the level setting
- compress=NAME:N - if N is in the allowed range for NAME then take it

The syntax is consistent for all three compressions.