linux-kernel - Re: [PATCH 03/10] soft-offline: use migrate_pages() instead of migrate_huge

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Date:	Wed, 27 Mar 2013 14:00:18 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/10] soft-offline: use migrate_pages() instead of
 migrate_huge_page()

On Tue 26-03-13 16:35:35, Naoya Horiguchi wrote:
[...]
> The differences is that migrate_huge_page() has one hugepage as an argument,
> and migrate_pages() has a pagelist with multiple hugepages.
> I already told this before and I'm not sure it's enough to answer the question,
> so I explain another point about why this patch do like it.

OK, I am blind. It is
+       list_move(&hpage->lru, &pagelist);
+       ret = migrate_pages(&pagelist, new_page, MPOL_MF_MOVE_ALL,
+                               MIGRATE_SYNC, MR_MEMORY_FAILURE);

which moves it from active_list and so you have to put it back.

> I think that we must do putback_*pages() for source pages whether migration
> succeeds or not.
> But when we call migrate_pages() with a pagelist,
> the caller can't access to the successfully migrated source pages
> after migrate_pages() returns, because they are no longer on the pagelist.
> So putback of the successfully migrated source pages should be done *in*
> unmap_and_move() and/or unmap_and_move_huge_page().

If the migration succeeds then the page becomes unused and free after
its last reference drops. So I do not see any reason to put it back to
active list and free it right afterwards.
On the other hand unmap_and_move does the same thing (although page
reference counting is a bit more complicated in that case) so it would
be good to keep in sync with regular pages case.

> And when we used migrate_huge_page(), we passed a hugepage to be migrated
> as an argument, so the caller can still access to the page even if the
> migration succeeds.
[...]
-- 
Michal Hocko
SUSE Labs
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