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Date:   Wed, 11 Apr 2018 12:27:39 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Zi Yan <zi.yan@sent.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: shmem: enable thp migration (Re: [PATCH v1] mm:
 consider non-anonymous thp as unmovable page)
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On Wed, 11 Apr 2018 11:26:11 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Fri 06-04-18 03:07:11, Naoya Horiguchi wrote:
> > >From e31ec037701d1cc76b26226e4b66d8c783d40889 Mon Sep 17 00:00:00 2001
> > From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Date: Fri, 6 Apr 2018 10:58:35 +0900
> > Subject: [PATCH] mm: enable thp migration for shmem thp
> > 
> > My testing for the latest kernel supporting thp migration showed an
> > infinite loop in offlining the memory block that is filled with shmem
> > thps.  We can get out of the loop with a signal, but kernel should
> > return with failure in this case.
> > 
> > What happens in the loop is that scan_movable_pages() repeats returning
> > the same pfn without any progress. That's because page migration always
> > fails for shmem thps.
> > 
> > In memory offline code, memory blocks containing unmovable pages should
> > be prevented from being offline targets by has_unmovable_pages() inside
> > start_isolate_page_range().
> >
> > So it's possible to change migratability
> > for non-anonymous thps to avoid the issue, but it introduces more complex
> > and thp-specific handling in migration code, so it might not good.
> > 
> > So this patch is suggesting to fix the issue by enabling thp migration
> > for shmem thp. Both of anon/shmem thp are migratable so we don't need
> > precheck about the type of thps.
> > 
> > Fixes: commit 72b39cfc4d75 ("mm, memory_hotplug: do not fail offlining too early")
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Cc: stable@vger.kernel.org # v4.15+
> 
> I do not really feel qualified to give my ack but this is the right
> approach for the fix. We simply do expect that LRU pages are migrateable
> as well as zone_movable pages.
> 
> Andrew, do you plan to take it (with Kirill's ack).
> 

Sure.  What happened with "Michal's fix in another email"
(https://lkml.kernel.org/r/20180406051452.GB23467@hori1.linux.bs1.fc.nec.co.jp)?