linux-kernel - Re: question on NUMA page migration

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Date:	Sun, 21 Oct 2012 10:39:28 +0800
From:	Ni zhan Chen <nizhan.chen@...il.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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 11:53 PM, Rik van Riel wrote:
> Hi Andrea, Peter,
>
> I have a question on page refcounting in your NUMA
> page migration code.
>
> In Peter's case, I wonder why you introduce a new
> MIGRATE_FAULT migration mode. If the normal page
> migration / compaction logic can do without taking
> an extra reference count, why does your code need it?

Hi Rik van Riel,

This is which part of codes? Why I can't find MIGRATE_FAULT in latest 
v3.7-rc2?

Regards,
Chen

>
> In Andrea's case, we have a comment suggesting an
> extra refcount is needed, immediately followed by
> a put_page:
>
>         /*
>          * Pin the head subpage at least until the first
>          * __isolate_lru_page succeeds (__isolate_lru_page pins it
>          * again when it succeeds). If we unpin before
>          * __isolate_lru_page successd, the page could be freed and
>          * reallocated out from under us. Thus our previous checks on
>          * the page, and the split_huge_page, would be worthless.
>          *
>          * We really only need to do this if "ret > 0" but it doesn't
>          * hurt to do it unconditionally as nobody can reference
>          * "page" anymore after this and so we can avoid an "if (ret >
>          * 0)" branch here.
>          */
>         put_page(page);
>
> This also confuses me.
>
> If we do not need the extra refcount (and I do not
> understand why NUMA migrate-on-fault needs one more
> refcount than normal page migration), we can get
> rid of the MIGRATE_FAULT mode.
>
> If we do need the extra refcount, why is normal
> page migration safe? :)
>

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