linux-kernel - Re: [PATCH] hibernate / memory hotplug: always use for_each_populated

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Date:	Wed, 22 Jul 2009 19:49:55 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Nigel Cunningham <ncunningham@...a.org.au>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>, linux-mm@...ck.org
Subject: Re: [PATCH] hibernate / memory hotplug: always use for_each_populated_zone()

On Wednesday 22 July 2009, KAMEZAWA Hiroyuki wrote:
> On Tue, 21 Jul 2009 16:11:08 +0200
> "Rafael J. Wysocki" <rjw@...k.pl> wrote:
[...]
> Adding function like this is not very hard.
> 
> bool can_access_physmem(unsigned long pfn)
> {
> 	 char byte;
> 	 char *pg = __va(pfn << PAGE_SHIFT);
> 	 return (__get_user(byte, pg) == 0)
> }

Unfortunately we can't use __get_user() for hibernation, because it may sleep.
Some other mechanism would be necessary, it seems.

> and enough simple. But this may allow you to access remapped device's memory...
> Then, some range check will be required anyway.
> Can we detect io-remapped range from memmap or any ?
> (I think we'll have to skip PG_reserved page...)
> 
> > > Alternative is making use of walk_memory_resource() as memory hotplug does.
> > > It checks resource information registered.
> > 
> > I'd be fine with any _simple_ mechanism allowing us to check whether there's
> > a physical page frame for given page (or given PFN).
> > 
> 
> walk_memory_resource() is enough _simple_,  IMHO.
> Now, I'm removing #ifdef CONFIG_MEMORY_HOTPLUG for walk_memory_resource() to
> rewrite /proc/kcore. 

Hmm.  Which architectures set CONFIG_ARCH_HAS_WALK_MEMORY ?

Best,
Rafael
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