linux-kernel - Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

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Message-ID: <20171011081317.b2cdnhjdyzgdo3up@dhcp22.suse.cz>
Date:   Wed, 11 Oct 2017 10:13:17 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

On Wed 11-10-17 10:04:39, Vlastimil Babka wrote:
> On 10/11/2017 08:51 AM, Michal Hocko wrote:
[...]
> > This is really strange! As you write in other email the page is
> > reserved. That means that some of the earlier checks 
> > 	if (zone_idx(zone) == ZONE_MOVABLE)
> > 		return false;
> > 	mt = get_pageblock_migratetype(page);
> > 	if (mt == MIGRATE_MOVABLE || is_migrate_cma(mt))
> 
> The MIGRATE_MOVABLE check is indeed bogus, because that doesn't
> guarantee there are no unmovable pages in the block (CMA block OTOH
> should be a guarantee).

OK, thanks for confirmation. I will remove the MIGRATE_MOVABLE check
here. Do you think it is worth removing CMA check as well? This is
merely an optimization AFAIU because we do not have to check the full
pageblockworth of pfns.

Anyway, let's way for Michael to confirm it really helps. If yes I will
post a full patch and ask Andrew to add it as a prerequisite for this
patch when sending to Linus to prevent the regression.
-- 
Michal Hocko
SUSE Labs