linux-kernel - Re: [PATCH] Make CONFIG_MIGRATION available for s390

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Date:	Mon, 07 Jul 2008 19:28:09 +0200
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Andy Whitcroft <apw@...dowen.org>
Subject: Re: [PATCH] Make CONFIG_MIGRATION available for s390

On Mon, 2008-07-07 at 10:09 -0500, Christoph Lameter wrote:
> This will extend the number of pages that are migratable and lead to strange
> semantics in the NUMA case. There suddenly vma_is migratable will forbid hotplug
> to migrate certain pages. 
> 
> I think we need two functions:
> 
> vma_migratable()	General migratability
> 
> vma_policy_migratable()	Migratable under NUMA policies.

Nothing will change here for the NUMA case, this is all about making it
compile w/o NUMA and with MIGRATION. What new strange semantics do you mean?
BTW, the latest patch in this thread will not touch vma_migratable() anymore,
I haven't read your mail before, sorry.

> That wont work since the migrate function takes a nodemask! The point of
> the function is to move memory from node to node which is something that you
> *cannot* do in a non NUMA configuration. So leave this chunk out.

Right, but I noticed that this function definition was needed to make it
compile with MIGRATION and w/o NUMA, although it would never be called in
non-NUMA config.
A better solution would probably be to put migrate_vmas(), the only caller
of vm_ops->migrate(), inside '#ifdef CONFIG_NUMA', because it will only be
called from NUMA-only mm/mempolicy.c. Does that sound reasonable?

> Hmmm... Okay. I tried to make MIGRATION as independent of CONFIG_NUMA as possible so hopefully this will work.

Umm, it doesn't compile with MIGRATION and w/o NUMA, which was the reason
for this patch, because of the policy_zone reference in vma_migratable()
and the missing vm_ops->migrate() function.

Thanks,
Gerald

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