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

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Date:	Fri, 29 Mar 2013 10:56:00 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, 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()

Michal Hocko <mhocko@...e.cz> writes:

> On Tue 26-03-13 16:59:40, Aneesh Kumar K.V wrote:
>> Naoya Horiguchi <n-horiguchi@...jp.nec.com> writes:
> [...]
>> > diff --git v3.9-rc3.orig/mm/memory-failure.c v3.9-rc3/mm/memory-failure.c
>> > index df0694c..4e01082 100644
>> > --- v3.9-rc3.orig/mm/memory-failure.c
>> > +++ v3.9-rc3/mm/memory-failure.c
>> > @@ -1467,6 +1467,7 @@ static int soft_offline_huge_page(struct page *page, int flags)
>> >  	int ret;
>> >  	unsigned long pfn = page_to_pfn(page);
>> >  	struct page *hpage = compound_head(page);
>> > +	LIST_HEAD(pagelist);
>> >
>> >  	/*
>> >  	 * This double-check of PageHWPoison is to avoid the race with
>> > @@ -1482,12 +1483,20 @@ static int soft_offline_huge_page(struct page *page, int flags)
>> >  	unlock_page(hpage);
>> >
>> >  	/* Keep page count to indicate a given hugepage is isolated. */
>> > -	ret = migrate_huge_page(hpage, new_page, MPOL_MF_MOVE_ALL,
>> > -				MIGRATE_SYNC);
>> > -	put_page(hpage);
>> > +	list_move(&hpage->lru, &pagelist);
>> 
>> we use hpage->lru to add the hpage to h->hugepage_activelist. This will
>> break a hugetlb cgroup removal isn't it ?
>
> This particular part will not break removal because
> hugetlb_cgroup_css_offline loops until hugetlb_cgroup_have_usage is 0.
>
> Little bit offtopic:
> Btw. hugetlb migration breaks to charging even before this patchset
> AFAICS. The above put_page should remove the last reference and then it
> will uncharge it but I do not see anything that would charge a new page.
> This is all because regula LRU pages are uncharged when they are
> unmapped. But this a different story not related to this series.


But when we call that put_page, we would have alreayd move the cgroup
information to the new page. We have

	h_cg = hugetlb_cgroup_from_page(oldhpage);
	set_hugetlb_cgroup(oldhpage, NULL);

	/* move the h_cg details to new cgroup */
	set_hugetlb_cgroup(newhpage, h_cg);


in hugetlb_cgroup_migrate

-aneesh

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