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

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Date:   Wed, 11 Oct 2017 13:17:13 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
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 10/11/2017 10:13 AM, Michal Hocko wrote:
> 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.

Actually, we should remove the CMA part as well. It's true that
MIGRATE_CMA does guarantee that the *buddy allocator* won't allocate
non-MOVABLE pages from the pageblock. But if the memory got allocated as
an actual CMA allocation (alloc_contig...) it will almost certainly not
be movable.

> 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.
>