Re: question on NUMA page migration

From: Peter Zijlstra
Date: Fri Oct 19 2012 - 13:54:33 EST

On Fri, 2012-10-19 at 13:13 -0400, Rik van Riel wrote:

> 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...

Right, it appears to not do this, it gets pages from the pfn and
zone->lock and the isolate_lru_page() call is the first 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.

It'd have to be inside migrate_misplaced_page(), can't do before
isolate_lru_page() or the page might disappear. Doing it after is
(obviously) too late.

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

You didn't seem to mind too much earlier, but I can remove it if you
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