Date: Wed, 31 Jul 2013 16:24:09 -0400
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Mel Gorman <mgorman@suse.de>, Hugh Dickins <hughd@google.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andi Kleen <andi@firstfloor.org>, Hillf Danton <dhillf@gmail.com>,
        Michal Hocko <mhocko@suse.cz>, Rik van Riel <riel@redhat.com>,
        Wanpeng Li <liwanp@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org,
        Naoya Horiguchi <nao.horiguchi@gmail.com>
Message-ID: <1375302249-scfvftrh-mutt-n-horiguchi@ah.jp.nec.com>
In-Reply-To: <87k3k7q4ox.fsf@linux.vnet.ibm.com>
References: <1374728103-17468-1-git-send-email-n-horiguchi@ah.jp.nec.com>
 <1374728103-17468-9-git-send-email-n-horiguchi@ah.jp.nec.com>
 <87k3k7q4ox.fsf@linux.vnet.ibm.com>
Subject: Re: [PATCH 8/8] prepare to remove
 /proc/sys/vm/hugepages_treat_as_movable
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On Wed, Jul 31, 2013 at 12:02:30AM +0530, Aneesh Kumar K.V wrote:
> Naoya Horiguchi <n-horiguchi@ah.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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