From: Hugh Dickins <hughd@google.com>
To: Dave Jones <davej@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]
Date: Fri, 2 Nov 2012 16:26:03 -0700 (PDT) [thread overview]
Message-ID: <alpine.LNX.2.00.1211021606580.11106@eggly.anvils> (raw)
In-Reply-To: <20121102014336.GA1727@redhat.com>
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
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next prev parent reply other threads:[~2012-11-02 23:26 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-25 2:37 Dave Jones
2012-10-25 4:36 ` Hugh Dickins
2012-10-25 4:50 ` Ni zhan Chen
2012-10-25 6:59 ` Hugh Dickins
2012-10-25 9:53 ` Ni zhan Chen
2012-10-25 10:21 ` Ni zhan Chen
2012-10-25 21:27 ` Hugh Dickins
2012-10-26 1:48 ` Ni zhan Chen
2012-10-25 11:14 ` Dave Jones
2012-10-25 21:28 ` Hugh Dickins
2012-10-25 20:52 ` Johannes Weiner
2012-10-25 21:48 ` Hugh Dickins
2012-10-26 2:15 ` Ni zhan Chen
2012-11-01 19:10 ` Dave Jones
2012-11-01 23:03 ` Hugh Dickins
2012-11-01 23:20 ` Dave Jones
2012-11-01 23:48 ` Hugh Dickins
2012-11-02 1:43 ` Dave Jones
2012-11-02 23:26 ` Hugh Dickins [this message]
2012-11-06 1:32 ` [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON Hugh Dickins
2012-11-06 13:54 ` Dave Jones
2012-11-06 23:48 ` Hugh Dickins
2012-11-07 22:38 ` Dave Jones
2012-11-14 1:36 ` [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON fix Hugh Dickins
2012-11-14 3:07 ` [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON Jaegeuk Hanse
2012-11-14 3:50 ` Hugh Dickins
2012-11-14 6:14 ` Dave Jones
2012-11-14 10:06 ` Hugh Dickins
2012-11-15 7:39 ` Jaegeuk Hanse
2012-11-15 19:56 ` Hugh Dickins
2012-11-16 0:40 ` Jaegeuk Hanse
2012-11-16 9:34 ` Jaegeuk Hanse
2012-11-17 4:48 ` Hugh Dickins
2012-11-18 0:57 ` Jaegeuk Hanse
2012-11-18 1:48 ` Jaegeuk Hanse
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