lkml.org 
[lkml]   [2012]   [Nov]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]
On Thu, 1 Nov 2012, Dave Jones wrote:
> On Thu, Nov 01, 2012 at 04:48:41PM -0700, Hugh Dickins wrote:
> >
> > Fedora turns on CONFIG_DEBUG_VM?
>
> Yes.
>
> > All mm developers should thank you for the wider testing exposure;
> > but I'm not so sure that Fedora users should thank you for turning
> > it on - really it's for mm developers to wrap around !assertions or
> > more expensive checks (e.g. checking calls) in their development.
>
> The last time I did some benchmarking the impact wasn't as ridiculous
> as say lockdep, or spinlock debug.

I think you're safe to assume that (outside of an individual developer's
private tree) it will never be nearly as heavy as lockdep or debug
pagealloc. I hadn't thought of spinlock debug as a heavy one, but
yes, I guess it would be heavier than almost all VM_BUG_ON()s.

> Maybe the benchmarks I was using
> weren't pushing the VM very hard, but it seemed to me that the value
> in getting info in potential problems early was higher than a small
> performance increase.

We thank you. I may have been over-estimating how much we put inside
those VM_BUG_ON()s, sorry. Just so long as you're aware that there's
a danger that one day we might slip something heavier in there.

Those few explicit #ifdef CONFIG_DEBUG_VMs sometimes found in mm/
are probably the worst: you might want to check on the current crop.

>
> > Or did I read a few months ago that some change had been made to
> > such definitions, and VM_BUG_ON(contents) are evaluated even when
> > the config option is off? I do hope I'm mistaken on that.
>
> Pretty sure that isn't the case. I remember Andrew chastising people
> a few times for putting checks in VM_BUG_ON's that needed to stay around
> even when the config option was off. Perhaps you were thinking of one
> of those incidents ?

Avoiding side-effects in BUG_ON and VM_BUG_ON. Yes, that comes up
from time to time, and I'm a believer on that. I think the discussion
I'm mis/remembering sprung out of one of those: someone was surprised
by the disassembly they found when it was configured off.

The correct answer is to try it for myself and see. Not today.

Hugh


\
 
 \ /
  Last update: 2012-11-03 00:41    [W:0.646 / U:0.088 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site