linux-kernel - Re: [PATCH 1/2] mm: drop migrate type checks from has_unmovable

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Date:   Thu, 19 Oct 2017 11:51:11 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Michael Ellerman <mpe@...erman.id.au>,
        Vlastimil Babka <vbabka@...e.cz>,
        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>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 1/2] mm: drop migrate type checks from has_unmovable_pages

On Fri, Oct 13, 2017 at 02:00:12PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.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.