linux-kernel - Re: BUG: Bad page state [was: Strange oopses in 2.6.30]

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Date:	Mon, 29 Jun 2009 11:37:06 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Jiri Slaby <jirislaby@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>
Subject: Re: BUG: Bad page state [was: Strange oopses in 2.6.30]

On Mon, Jun 29, 2009 at 12:18:19PM +0200, Johannes Weiner wrote:
> On Mon, Jun 29, 2009 at 09:41:14AM +0100, Mel Gorman wrote:
> > I see the unconditionoal clearing of the flag was merged since but even
> > that might be too heavy handed as we are making a locked bit operation
> > on every page free. That's unfortunate overhead to incur on every page
> > free to handle a situation that should not be occurring at all.
> 
> Linus was probably quick to merge it as istr several people hitting
> bad_page() triggering.  We should get rid of the locked op, I was just
> not 100% sure and chose the safer version.
> 

And I have no problem with the decision. Leaving it as it was would have
caused a storm of bug reports, all similar.

> > > > +		WARN_ONCE(1, KERN_WARNING
> > > > +			"Sloppy page flags set process %s at pfn:%05lx\n"
> > > > +			"page:%p flags:%p\n",
> > > > +			current->comm, page_to_pfn(page),
> > > > +			page, (void *)page->flags);
> [...]
> > > > +		page->flags &= ~PAGE_FLAGS_WARN_AT_FREE;
> > > > +	}
> > > > +
> > > >  	if (unlikely(page_mapcount(page) |
> > > >  		(page->mapping != NULL)  |
> > > >  		(atomic_read(&page->_count) != 0) |
> > > 
> > > Howerver, I like this patch concept. this warning is useful and meaningful IMHO.
> > > 
> > 
> > This is a version that is based on top of current mainline that just
> > displays the warning. However, I think we should consider changing
> > TestClearPageMlocked() back to PageMlocked() and only clearing the flags
> > when the unusual condition is encountered.
> 
> I have a diff at home that makes this an unlocked
> __TestClearPageMlocked(), would you be okay with this?
> 

It'd be an improvement for sure. Post it and I'll take a look.

My preference is still to clear the flag only when found to be erroneously set
and print a warning once but that's because it was the patch I put together
so I'm biased :)

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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