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
next prev parent reply other threads:[~2012-11-02 23:26 UTC|newest]
Thread overview: 34+ 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 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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