linux-kernel - Re: [PATCH 8/8] prepare to remove /proc/sys/vm/hugepages_treat_as

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Date:	Wed, 31 Jul 2013 16:24:09 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, Michal Hocko <mhocko@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH 8/8] prepare to remove
 /proc/sys/vm/hugepages_treat_as_movable

On Wed, Jul 31, 2013 at 12:02:30AM +0530, Aneesh Kumar K.V wrote:
> Naoya Horiguchi <n-horiguchi@...jp.nec.com> writes:
> 
> > Now hugepages are definitely movable. So allocating hugepages from
> > ZONE_MOVABLE is natural and we have no reason to keep this parameter.
> > In order to allow userspace to prepare for the removal, let's leave
> > this sysctl handler as noop for a while.
> 
> I guess you still need to handle architectures for which pmd_huge is
> 
> int pmd_huge(pmd_t pmd)
> {
> 	return 0;
> }
> 
> embedded powerpc is one. They don't store pte information at the PMD
> level. Instead pmd contains a pointer to hugepage directory which
> contain huge pte.

It seems that this comment is for the whole series, not just for this
patch, right?

Some users of hugepage migration (mbind, move_pages, migrate_pages)
walk over page tables to collect hugepages to be migrated, where
hugepages are just ignored in such architectures due to pmd_huge.
So no problem for these users.

But the other users (softoffline, memory hotremove) choose hugepages
to be migrated based on pfn, where they don't check pmd_huge.
As you wrote, this can be problematic for such architectures.
So I think of adding pmd_huge() check somewhere (in unmap_and_move_huge_page
for example) to make it fail for such architectures.

Thanks,
Naoya Horiguchi
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