linux-kernel - Re: shmem_getpage_gfp VM_BUG

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Message-ID: <alpine.LNX.2.00.1211011627120.19567@eggly.anvils>
Date:	Thu, 1 Nov 2012 16:48:41 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Dave Jones <davej@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

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?

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.

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.

Hugh
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