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SubjectRe: 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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