linux-kernel - Re: shmem_getpage_gfp VM_BUG

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Message-ID: <20121102014336.GA1727@redhat.com>
Date:	Thu, 1 Nov 2012 21:43:36 -0400
From:	Dave Jones <davej@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

On Thu, Nov 01, 2012 at 04:48:41PM -0700, Hugh Dickins wrote:
 > On Thu, 1 Nov 2012, Dave Jones wrote:
 > > On Thu, Nov 01, 2012 at 04:03:40PM -0700, Hugh Dickins wrote:
 > >  > 
 > >  > Except... earlier in the thread you explained how you hacked
 > >  > #define VM_BUG_ON(cond) WARN_ON(cond)
 > >  > to get this to come out as a warning instead of a bug,
 > >  > and now it looks as if "a user" has here done the same.
 > >  > 
 > >  > Which is very much a user's right, of course; but does
 > >  > make me wonder whether that user might actually be davej ;)
 > > 
 > > indirectly. I made the same change in the Fedora kernel a while ago
 > > to test a hypothesis that we weren't getting any VM_BUG_ON reports.
 > 
 > 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. 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.

 > 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 ?

	Dave
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