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Date:   Fri, 6 Apr 2018 09:08:15 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Zi Yan <zi.yan@sent.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.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 Fri 06-04-18 05:14:53, Naoya Horiguchi wrote:
> On Fri, Apr 06, 2018 at 03:07:11AM +0000, Horiguchi Naoya(堀口 直也) 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+
> 
> ... oh, I don't think this is suitable for stable.
> Michal's fix in another email can come first with "CC: stable",
> then this one.
> Anyway I want to get some feedback on the change of this patch.

My patch is indeed much simpler but it depends on [1] and that doesn't
sound like a stable material as well because it depends on onether 2
patches. Maybe we need some other hack for 4.15 if we really care enough.

[1] http://lkml.kernel.org/r/20180103082555.14592-4-mhocko@kernel.org
-- 
Michal Hocko
SUSE Labs