linux-kernel - Re: [PATCH -mm][BUGFIX] migration_entry

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Date:	Wed, 18 Jun 2008 16:26:09 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	"hugh@...itas.com" <hugh@...itas.com>
Subject: Re: [PATCH -mm][BUGFIX] migration_entry_wait fix. v2

> In speculative page cache look up protocol, page_count(page) is set to 0
> while radix-tree modification is going on, truncation, migration, etc...
> 
> While page migration, a page fault to page under migration does
>  - look up page table
>  - find it is migration_entry_pte
>  - decode pfn from migration_entry_pte and get page of pfn_page(pfn)
>  - wait until page is unlocked 
> 
> It does get_page() -> wait_on_page_locked() -> put_page() now.
> 
> In page migration's radix-tree replacement, page_freeze_refs() ->
> page_unfreeze_refs() is called. And page_count(page) turns to be zero
> and must be kept to be zero while radix-tree replacement.
> 
> If get_page() is called against a page under radix-tree replacement,
> the kernel panics(). To avoid this, we shouldn't increment page_count()
> if it is zero. This patch uses get_page_unless_zero().
> 
> Even if get_page_unless_zero() fails, the caller just retries.
> But will be a bit busier.

Great!
	Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>


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