linux-kernel - Re: [PATCH] migration_entry

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Message-Id: <20080618150436.dca5eb75.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 18 Jun 2008 15:04:36 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Kosaki Motohiro <kosaki.motohiro@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	"hugh@...itas.com" <hugh@...itas.com>
Subject: Re: [PATCH] migration_entry_wait fix.

On Wed, 18 Jun 2008 15:35:57 +1000
Nick Piggin <nickpiggin@...oo.com.au> wrote:

> On Wednesday 18 June 2008 11:54, KAMEZAWA Hiroyuki wrote:
> > On Wed, 18 Jun 2008 10:13:49 +0900
> >
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > > +	if (!page_cache_get_speculative())
> > > +		goto out;
> >
> > This is obviously buggy....sorry..quilt refresh miss..
> >
> > ==
> > In speculative page cache lookup protocol, page_count(page) is set to 0
> > while radix-tree modification is going on, truncation, migration, etc...
> 
> These tend to all happen while the page is locked, and in particular
> while the page does not have any references other than the current
> code path and the pagecache. So no page tables should point to it.
> 
> So migration_entry_wait should not find pages with a refcount of zero.
> 
> 
> > While page migration, a page fault to page under migration should wait
> > unlock_page() and migration_entry_wait() waits for the page from its
> > pte entry. It does get_page() -> wait_on_page_locked() -> put_page() now.
> >
> > In page migration, page_freeze_refs() -> page_unfreeze_refs() is called.
> >
> > Here, page_unfreeze_refs() expects page_count(page) == 0 and panics
> > if page_count(page) != 0. To avoid this, we shouldn't touch page_count()
> > if it is zero. This patch uses page_cache_get_speculative() to avoid
> > the panic.
> 
> At any rate, page_cache_get_speculative() should not be used for this
> purpose, but for when we _really_ don't have any references to a page.
> 
Then, I got NAK. what should I do ? 
(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().

Maybe we just go back to user-land and makes it to do page-fault again
is better ? 
 
Thanks,
-Kame










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