linux-kernel - Re: question on NUMA page migration

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Message-ID: <50818A41.7030909@redhat.com>
Date:	Fri, 19 Oct 2012 13:13:37 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Mel Gorman <mel@....ul.ie>,
	Linux kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: question on NUMA page migration

On 10/19/2012 12:39 PM, Peter Zijlstra wrote:
> On Fri, 2012-10-19 at 11:53 -0400, Rik van Riel wrote:
>>
>> If we do need the extra refcount, why is normal
>> page migration safe? :)
>
> Its mostly a matter of how convoluted you make the code, regular page
> migration is about as bad as you can get
>
> Normal does:
>
>    follow_page(FOLL_GET) +1
>
>    isolate_lru_page() +1
>
>    put_page() -1
>
> ending up with a page with a single reference (for anon, or one extra
> each for the mapping and buffer).

Would it make sense to have the normal page migration code always
work with the extra refcount, so we do not have to introduce a new
MIGRATE_FAULT migration mode?

On the other hand, compaction does not take the extra reference...

Another alternative might be to do the put_page inside
do_prot_none_numa().  That would be analogous to do_wp_page
disposing of the old page for the caller.

I am not real happy about NUMA migration introducing its own
migration mode...

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