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Date:   Wed, 11 Apr 2018 21:43:26 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.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)
Message-ID: <20180411194326.GN23400@dhcp22.suse.cz>
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On Wed 11-04-18 12:27:39, Andrew Morton wrote:
> 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)?

I guess you meant http://lkml.kernel.org/r/20180405190405.GS6312@dhcp22.suse.cz

Well, that would be a workaround in case we didn't have a proper fix. It
is much simpler but it wouldn't make backporting to older kernels any
easier because it depends on other non-trivial changes you already have
in your tree. So having a full THP pagecache migration support is
preferred of course.

-- 
Michal Hocko
SUSE Labs