linux-kernel - question on NUMA page migration

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Message-ID: <5081777A.8050104@redhat.com>
Date:	Fri, 19 Oct 2012 11:53:30 -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: question on NUMA page migration

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?

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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