Re: [PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in 2.6.26-rc5-mm3

[Nick Piggin]

On Wednesday 18 June 2008 05:28, Lee Schermerhorn wrote:
> On Tue, 2008-06-17 at 19:33 +0100, Hugh Dickins wrote:
> > On Tue, 17 Jun 2008, Lee Schermerhorn wrote:
> > > Now I wonder if the assertion that newpage count == 1 could be
> > > violated? I don't see how.  We've just allocated and filled it and
> > > haven't unlocked it yet, so we should hold the only reference.  Do you
> > > agree?
> >
> > Disagree: IIRC, excellent example of the kind of assumption
> > that becomes invalid with Nick's speculative page references.
> >
> > Someone interested in the previous use of the page may have
> > incremented the refcount, and in due course will find that
> > it's got reused for something else, and will then back off.
>
> Yeah.  Kosaki-san mentioned that we'd need some rework for the
> speculative page cache work.  Looks like we'll need to drop the
> VM_BUG_ON().
>
> I need to go read up on the new invariants we can trust with the
> speculative page cache.

I don't know if I've added a summary, which is something I should
do.

The best thing to do is never use page_count, but just use get
and put to refcount it. If you really must use it:

- If there are X references to a page, page_count will return >= X.
- If page_count returns Y, there are no more than Y references to the page.

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