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Date:   Tue, 3 Apr 2018 12:58:15 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.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 v1] mm: consider non-anonymous thp as unmovable page
Message-ID: <20180403105815.GL5501@dhcp22.suse.cz>
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On Tue 03-04-18 13:54:11, Kirill A. Shutemov wrote:
> On Tue, Apr 03, 2018 at 10:34:51AM +0200, Michal Hocko wrote:
> > On Tue 03-04-18 08:24:06, Naoya Horiguchi wrote:
> > > On Tue, Apr 03, 2018 at 09:59:28AM +0200, Michal Hocko wrote:
> > > > On Tue 03-04-18 13:46:28, Naoya Horiguchi wrote:
> > > > > My testing for the latest kernel supporting thp migration found out 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.
> > > >
> > > > Why does it fail? Shmem pages should be movable without any issues.
> > > 
> > > .. because try_to_unmap_one() explicitly skips unmapping for migration.
> > > 
> > >   #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> > >                   /* PMD-mapped THP migration entry */
> > >                   if (!pvmw.pte && (flags & TTU_MIGRATION)) {
> > >                           VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);
> > >   
> > >                           if (!PageAnon(page))
> > >                                   continue;
> > >   
> > >                           set_pmd_migration_entry(&pvmw, page);
> > >                           continue;
> > >                   }
> > >   #endif
> > > 
> > > When I implemented this code, I felt hard to work on both of anon thp
> > > and shmem thp at one time, so I separated the proposal into smaller steps.
> > > Shmem uses pagecache so we need some non-trivial effort (including testing)
> > > to extend thp migration for shmem. But I think it's a reasonable next step.
> > 
> > OK, I see. I have forgot about this part. Please be explicit about that
> > in the changelog. Also the proper fix is to not use movable zone for
> > shmem page THP rather than hack around it in the hotplug specific code
> > IMHO.
> 
> No. We should just split the page before running
> try_to_unmap(TTU_MIGRATION) on the page.

If splitting is a preffered way then I do not have any objection. We
just cannot keep unmovable objects in the zone movable.
-- 
Michal Hocko
SUSE Labs