Message-Id: <1225129574.3199.82.camel@calx>
Date:	Mon, 27 Oct 2008 12:46:14 -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 Mon, 2008-10-27 at 12:50 -0400, Chris Snook wrote:
> Matt Mackall wrote:
> > 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?
> 
> Because the 1024 limit is a created by an array declaration in printk.c, which 
> uses the number 1024.  Instead, put something like this in a header file:
> 
> #define PRINTK_MAX 1024

That 1024 is supposed to effectively be infinity. Printk is intended to
be a largely line-oriented facility. If you're printing something large
enough that you even have to ask yourself 'hmm, how big a buffer can I
print?', you're printing something -way- too large.

So no, I think printk is fine as it stands.

> Arbitrarily clamping critical debugging features to limits pulled out of one's 
> ass is generally frowned upon.

First, I don't think it's accurate to apply the adjective 'critical' to
a facility that's existed in mainline for mere days, while sysfs itself
has existed many years.

Second, I didn't pull it out of my ass, actually. I did a histogram of
sysfs path lengths on my system, found it cut off at ~100, and then
added generous padding.

>   If you can justify some limit lower than the 
> length of the printk buffer, great, but "I want to save a few hundred bytes of 
> RAM, total" is insufficient, unless you want to restrict it to the 
> CONFIG_EMBEDDED world.

I think you've got this exactly backwards. This is just a debugging
hint. It is not a standards correctness or kernel robustness issue.
Reporting the last 20 bytes of the filename would be a sufficient hint
for most purposes. On the vast majority of machines, this feature will
be effectively dormant, and this buffer will be wasted space so we have
to weigh the usefulness of storing unusually long filenames against the
value of that space for -all other purposes-. If it's going to be on
unconditionally, it should be as small as possible.

Just for fun, let's imagine for a moment that someone -did- have a 1k
sysfs path. And, further, we happen to have a fatal oops at some point
after it. And, further, we were lucky enough for it to get directed to a
text console. But unfortunately, our debugging hint has helpfully taken
up half the screen, and we've actually scrolled the much more important
*cause of the oops* off the screen. Hurray! In fact, we'll probably
scroll most of the giant sysfs path off too when we print a backtrace. 

Given that the only purpose of this feature is to be printked in an
already tightly constrained oops report where it's quite likely to be
completely extraneous, spending more than a couple lines on it is a good
way to fail.

-- 
Mathematics is the supreme nostalgia of our time.

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