linux-kernel - Re: [PATCH 1/2] debugfs: add small file operations for most files

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Message-ID: <e3c39c3e7be70993c7fa07e58c2beda52ff44cb9.camel@sipsolutions.net>
Date: Tue, 22 Oct 2024 16:02:09 +0200
From: Johannes Berg <johannes@...solutions.net>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org, Alexander
 Viro <viro@...iv.linux.org.uk>, "Rafael J . Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH 1/2] debugfs: add small file operations for most files

On Tue, 2024-10-22 at 15:55 +0200, Greg Kroah-Hartman wrote:
> On Tue, Oct 22, 2024 at 03:18:34PM +0200, Johannes Berg wrote:
> > From: Johannes Berg <johannes.berg@...el.com>
> > 
> > As struct file_operations is really big, but (most) debugfs
> > files only use simple_open, read, write and perhaps seek, and
> > don't need anything else, this wastes a lot of space for NULL
> > pointers.
> > 
> > Add a struct debugfs_short_fops and some bookkeeping code in
> > debugfs so that users can use that with debugfs_create_file()
> > using _Generic to figure out which function to use.
> > 
> > Converting mac80211 to use it where possible saves quite a
> > bit of space:
> > 
> > 1010127  205064    1220 1216411  128f9b net/mac80211/mac80211.ko (before)
> >  981199  205064    1220 1187483  121e9b net/mac80211/mac80211.ko (after)
> > -------
> >  -28928 = ~28KiB
> > 
> > With a marginal space cost in debugfs:
> > 
> >    8701	    550	     16	   9267	   2433	fs/debugfs/inode.o (before)
> >   25233	    325	     32	  25590	   63f6	fs/debugfs/file.o  (before)
> >    8914	    558	     16	   9488	   2510	fs/debugfs/inode.o (after)
> >   25380	    325	     32	  25737	   6489	fs/debugfs/file.o  (after)
> > ---------------
> >    +360      +8
> > 
> > (All on x86-64)
> > 
> > A simple spatch suggests there are more than 300 instances,
> > not even counting the ones hidden in macros like in mac80211,
> > that could be trivially converted, for additional savings of
> > about 240 bytes for each.
> > 
> > Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> 
> I imagine you want to take this through the wireless tree for the second
> patch, so feel free to do that and add:

I don't even really care, we're not likely to be changing the mac80211
debugfs code in a way that'd create (significant) conflicts. But I can
do that, thanks!

johannes