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

    Something like this or it is completely broken. I completely forgot the
    whole page_vma_mapped_walk business.

    diff --git a/mm/rmap.c b/mm/rmap.c
    index 9eaa6354fe70..cbbfbcb08b83 100644
    --- a/mm/rmap.c
    +++ b/mm/rmap.c
    @@ -1356,6 +1356,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
    return true;

    if (flags & TTU_SPLIT_HUGE_PMD) {
    +split:
    split_huge_pmd_address(vma, address,
    flags & TTU_SPLIT_FREEZE, page);
    }
    @@ -1375,7 +1376,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
    VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);

    if (!PageAnon(page))
    - continue;
    + goto split;

    set_pmd_migration_entry(&pvmw, page);
    continue;
    --
    Michal Hocko
    SUSE Labs
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