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Date: Thu, 19 Oct 2017 11:51:11 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, Michael Ellerman <mpe@ellerman.id.au>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Reza Arbab <arbab@linux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@gmail.com>, qiuxishi@huawei.com,
        Igor Mammedov <imammedo@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 1/2] mm: drop migrate type checks from has_unmovable_pages
Message-ID: <20171019025111.GA3852@js1304-P5Q-DELUXE>
References: <20171013115835.zaehapuucuzl2vlv@dhcp22.suse.cz>
 <20171013120013.698-1-mhocko@kernel.org>
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On Fri, Oct 13, 2017 at 02:00:12PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> Michael has noticed that the memory offline tries to migrate kernel code
> pages when doing
>  echo 0 > /sys/devices/system/memory/memory0/online
> 
> The current implementation will fail the operation after several failed
> page migration attempts but we shouldn't even attempt to migrate
> that memory and fail right away because this memory is clearly not
> migrateable. This will become a real problem when we drop the retry loop
> counter resp. timeout.
> 
> The real problem is in has_unmovable_pages in fact. We should fail if
> there are any non migrateable pages in the area. In orther to guarantee
> that remove the migrate type checks because MIGRATE_MOVABLE is not
> guaranteed to contain only migrateable pages. It is merely a heuristic.
> Similarly MIGRATE_CMA does guarantee that the page allocator doesn't
> allocate any non-migrateable pages from the block but CMA allocations
> themselves are unlikely to migrateable. Therefore remove both checks.

Hello,

This patch will break the CMA user. As you mentioned, CMA allocation
itself isn't migrateable. So, after a single page is allocated through
CMA allocation, has_unmovable_pages() will return true for this
pageblock. Then, futher CMA allocation request to this pageblock will
fail because it requires isolating the pageblock.

Thanks.