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

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Date:	Tue, 26 Mar 2013 10:49:50 +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 00:34:40, Naoya Horiguchi wrote:
> On Mon, Mar 25, 2013 at 01:31:28PM +0100, Michal Hocko wrote:
> > On Fri 22-03-13 16:23:48, Naoya Horiguchi wrote:
[...]
> > > @@ -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);
> > > +	ret = migrate_pages(&pagelist, new_page, MPOL_MF_MOVE_ALL,
> > > +				MIGRATE_SYNC, MR_MEMORY_FAILURE);
> > >  	if (ret) {
> > >  		pr_info("soft offline: %#lx: migration failed %d, type %lx\n",
> > >  			pfn, ret, page->flags);
> > > +		/*
> > > +		 * We know that soft_offline_huge_page() tries to migrate
> > > +		 * only one hugepage pointed to by hpage, so we need not
> > > +		 * run through the pagelist here.
> > > +		 */
> > > +		putback_active_hugepage(hpage);
> > 
> > Maybe I am missing something but why we didn't need to call this before
> > when using migrate_huge_page?
> 
> migrate_huge_page() does not need list handling before/after the call,
> because it's defined to migrate only one hugepage, and it has a page as
> an argument, not list_head.

I do not understand this reasoning. migrate_huge_page calls
unmap_and_move_huge_page and migrate_pages does the same + accounting.
So what is the difference here? I suspect that putback_active_hugepage
was simply missing in this code path.

> > > +		if (ret > 0)
> > > +			ret = -EIO;
> > >  	} else {
> > >  		set_page_hwpoison_huge_page(hpage);
> > >  		dequeue_hwpoisoned_huge_page(hpage);
> > > diff --git v3.9-rc3.orig/mm/migrate.c v3.9-rc3/mm/migrate.c
> > > index f69f354..66030b6 100644
> > > --- v3.9-rc3.orig/mm/migrate.c
> > > +++ v3.9-rc3/mm/migrate.c
> > > @@ -981,6 +981,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
> > >  
> > >  	unlock_page(hpage);
> > >  out:
> > > +	if (rc != -EAGAIN)
> > > +		putback_active_hugepage(hpage);
> > 
> > And why do you put it here? If it is called from migrate_pages then the
> > caller already does the clean-up (putback_lru_pages).
> 
> What the caller of migrate_pages() cleans up is the (huge)pages which failed
> to be migrated. And what the above code cleans up is the source hugepage
> after the migration succeeds.

Why should you want to add successfully migrated page? /me confused.

-- 
Michal Hocko
SUSE Labs
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