Re: [PATCH] migration_entry_wait fix.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 18 Jun 2008 16:42:37 +1000
Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> > (This fix is not related to lock_page() problem.)
> >
> > If I read your advice correctly, we shouldn't use lock_page() here.
> >
> > Before speculative page cache, page_table_entry of a page under migration
> > has a pte entry which encodes pfn as special pte entry. and wait for the
> > end of page migration by lock_page().
> 
> What I don't think I understand, is how we can have a page in the
> page tables (and with the ptl held) but with a zero refcount... Oh,
> it's not actually a page but a migration entry! I'm not quite so
> familiar with that code.
> 
> Hmm, so we might possibly see a page there that has a zero refcount
> due to page_freeze_refs? In which case, I think the direction of you
> fix is good. Sorry for my misunderstanding the problem, and thank
> you for fixing up my code!
> 
> I would ask you to use get_page_unless_zero rather than
> page_cache_get_speculative(), because it's not exactly a speculative
> reference -- a speculative reference is one where we elevate _count
> and then must recheck that the page we have is correct.
> 
ok.

> Also, please add a comment. It would really be nicer to hide this
> transiently-frozen state away from migration_entry_wait, but I can't
> see any lock that would easily solve it.
> 
ok, will adds comments.

Thanks,
-Kame


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