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

    Thanks,
    Naoya Horiguchi

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