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SubjectRe: [PATCH v1] mm: consider non-anonymous thp as unmovable page
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.

--
Kirill A. Shutemov

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