linux-kernel - Re: Bloatwatch 2.6.28-rc1: last_sysfs

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Message-Id: <1225123898.3199.20.camel@calx>
Date:	Mon, 27 Oct 2008 11:11:38 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	akpm <akpm@...uxfoundation.org>
Subject: Re: Bloatwatch 2.6.28-rc1: last_sysfs_file

On Sat, 2008-10-25 at 06:38 -0400, Chris Snook wrote: 
> Matt Mackall wrote:
> > On Fri, 2008-10-24 at 15:52 -0500, Matt Mackall wrote:
> >> 2.6.28-rc1 adds 4k for last_sysfs_file debug tracking. That's one hell
> >> of a long sysfs path.
> >>
> >> http://www.selenic.com/bloatwatch/?cmd=compare;v1=2.6.27;v2=2.6.28-rc1;part=/built-in/fs/sysfs
> > 
> > ..especially given that printk is limited to 1k at a time.
> > 
> > 
> > sysfs: shrink last_sysfs_file to a reasonable size
> > 
> > sysfs was reserving 4k to store filenames for debug despite printk being
> > limited to 1k. Shrink this to something more reasonable.
> > 
> > Signed-off-by: Matt Mackall <mpm@...enic.com>
> > 
> > diff -r ac8c82ff3be7 fs/sysfs/file.c
> > --- a/fs/sysfs/file.c	Fri Oct 24 13:13:04 2008 -0500
> > +++ b/fs/sysfs/file.c	Fri Oct 24 16:11:53 2008 -0500
> > @@ -25,7 +25,7 @@
> >  #include "sysfs.h"
> >  
> >  /* used in crash dumps to help with debugging */
> > -static char last_sysfs_file[PATH_MAX];
> > +static char last_sysfs_file[200]; /* allow for disgustingly long paths */
> >  void sysfs_printk_last_file(void)
> >  {
> >  	printk(KERN_EMERG "last sysfs file: %s\n", last_sysfs_file);
> > 
> > 
> 
> Please don't use magic numbers.  Use a symbolic constant, and modify printk.c to 
> use the same.

I'm explicitly not using a magic number because the relevant magic
number is absurdly large. It is in fact 4 times larger than printk can
print. And its 40 times larger than any path we are liable to encounter.
Inventing a new use-once #define meaning "good enough for debugging
99.99% of sysfs bugs" one line up is not an improvement here.

And why on earth would I modify anything in printk?

-- 
Mathematics is the supreme nostalgia of our time.

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