linux-kernel - Re: [RFC][PATCH 0/5] tracing: Add new file system tracefs

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Date:	Thu, 22 Jan 2015 10:15:30 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 0/5] tracing: Add new file system tracefs

On Thu, 22 Jan 2015 09:55:47 -0500
Tejun Heo <tj@...nel.org> wrote:

> kernfs provides two sets of file operations.  One is seq_file based
> and the other is direct read/write.  In both cases, bouncing data
> between userland and kernel is handled by kernfs.  If you already have
> existing read write ops implemented doing custom buffer handling and
> direct userland memory access, it'll take some adaptation but for a
> lot of cases this would consolidate duplicate code paths.

Does it also handle splice? That's a key part of the tracing code.

Almost every tracing file is somewhat unique. When things can be
shared, they are, but there's not much generic code that can be shared.

> 
> > I created tracefs with 700 lines of code and two files (inode.c and
> > tracefs.h), and for the users of tracefs, I just did
> > s/debugfs/tracefs/. If I can't make that substitution for the users,
> > that is a show stopper.
> > 
> > I don't see how I can use kernfs without it causing a lot of invasive
> > changes to the ftrace subsystem.
> 
> Converting an existing vfs based pseudo fs implementation over to
> kernfs isn't trivial.  I mean, if that were trivial, why would kernfs
> even exist?  kernfs is a layer which abstracts a large part of pseudo
> filesystem which provides extra features like significantly lower
> memory footprint with large number of nodes and revocation support in
> a way that its users, for the most part, hopefully, only have to worry
> about the content to provide to userland.

Sounds like some of the tracing files could benefit from this. But I'm
not sure kernfs has all the necessary features I need.

> 
> I frankly have no idea whether tracefs would be a good candidate for
> kernfs usage but if you're looking for a mechanical one-to-one
> conversion from vfs based implementation, that's not gonna work.

OK, thanks. Perhaps if tracing was still new I could have tried to go
with kernfs. But as debugfs was such a simple to use interface, it let
me concentrate more on the complexities of tracing itself instead of
spending time coming up with a complex interface.

If a one to one conversion to vfs is not gonna work, I'm going to be
interested in seeing how debugfs will be converted.

Anyway, I think I'm convinced that kernfs is not yet the way to go. I'm
going to continue on with my current path.

Thanks,

-- Steve
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