On Tue, Aug 17, 2010 at 11:40:08AM +0200, Andi Kleen wrote:
quoted text
> > When get_user_pages_fast() is called before try_to_unmap(),
> > direct I/O code increments refcount on the target page.
> > Because this refcount is not associated to the mapping,
> > migration code will find remaining refcounts after try_to_unmap()
> > unmaps all mappings. Then refcount check decides migration to fail,
> > so direct I/O is continued safely.
> 
> This would imply that direct IO can make migration fail arbitarily.
> Also not good. Should we add some retries, at least for the soft offline
> case?

Soft offline is kicked from userspace, so the retry logic can be implemented
in userspace. However, currently we can't distinguish migration failure from
"unknown page" error by return value (-EIO in both case for now.)
How about changing return value to -EAGAIN when migration failed?

Thanks,
Naoya Horiguchi
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