linux-kernel - Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

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Message-ID: <20190227212454.GA38679@dennisz-mbp.dhcp.thefacebook.com>
Date:   Wed, 27 Feb 2019 16:24:54 -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>,
        Nikolay Borisov <nborisov@...e.com>, kernel-team@...com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned
 up

On Wed, Feb 27, 2019 at 07:36:50PM +0100, David Sterba wrote:
> On Wed, Feb 27, 2019 at 01:29:16PM -0500, Dennis Zhou wrote:
> > > I've noticed while reading the code, why do you use the indirect call
> > > here? The wsm.ops points to btrfs_zstd_compress so free_workspace is
> > > always zstd_free_workspace.
> > > 
> > > The compiler is usually smart to replace such things by direct call if
> > > the type has not escaped, but this is not true for btrfs_compress_op so
> > > the indirect function call must be preserved.
> > 
> > I don't have a strong reason to use the indirect call here. It was just
> > to make it consistent for everyone to use the indirection. This at least
> > is in the cleanup path, so I don't think performance is that important?
> 
> It's not just that, the timer uses it too and there are indirect calls
> of the alloc_workspace callback. The indirection is not used by lzo nor
> zlib code, so I don't see what 'everyone' you mean. In the generic
> compression code it makes sense, I see that.
> 
> > But I don't feel strongly for or against calling zstd_free_workspace()
> > directly.
> 
> I feel strongly about not using the indirection when not necessary :)

Great :). I sent you a patch just now to remove the indirection [1].

[1] https://lore.kernel.org/linux-btrfs/20190227212128.38491-1-dennis@kernel.org

Thanks,
Dennis