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Date: Wed, 11 Apr 2012 14:12:44 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Glauber Costa <glommer@parallels.com>
Cc: <linux-kernel@vger.kernel.org>, <devel@openvz.org>, <linux-mm@kvack.org>,
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        Greg Thelen <gthelen@google.com>,
        Suleiman Souhlal <suleiman@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] remove BUG() in possible but rare condition
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On Wed, 11 Apr 2012 17:51:57 -0300
Glauber Costa <glommer@parallels.com> wrote:

> On 04/11/2012 05:26 PM, Andrew Morton wrote:
> >>
> >> >    failed:
> >> >  -	BUG();
> >> >    	unlock_page(page);
> >> >    	page_cache_release(page);
> >> >    	return NULL;
> > Cute.
> >
> > AFAICT what happened was that in my April 2002 rewrite of this code I
> > put a non-fatal buffer_error() warning in that case to tell us that
> > something bad happened.
> >
> > Years later we removed the temporary buffer_error() and mistakenly
> > replaced that warning with a BUG().  Only it*can*  happen.
> >
> > We can remove the BUG() and fix up callers, or we can pass retry=1 into
> > alloc_page_buffers(), so grow_dev_page() "cannot fail".  Immortal
> > functions are a silly fiction, so we should remove the BUG() and fix up
> > callers.
> >
> Any particular caller you are concerned with ?

Didn't someone see a buggy caller in btrfs?

I'm thinking that we should retain some sort of assertion (a WARN_ON)
if the try_to_free_buffers() failed.  This is a weird case which I
assume handles the situation where a blockdev's blocksize has changed. 
The code tries to throw away the old wrongly-sized buffer_heads and to
then add new correctly-sized ones.  If that discarding of buffers
fails then the kernel is in rather a mess.

It's quite possible that this code is never executed - we _should_ have
invalidated all the pagecache for that device when changing blocksize. 
Or maybe it *is* executed, I dunno.  It's one of those things which has
hung around for decades as code in other places has vastly changed.