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    SubjectRe: [PATCH 4/7] mm/hmm: properly handle migration pmd
    On Tue 28-08-18 11:36:59, Jerome Glisse wrote:
    > On Tue, Aug 28, 2018 at 05:24:14PM +0200, Michal Hocko wrote:
    > > On Fri 24-08-18 20:05:46, Zi Yan wrote:
    > > [...]
    > > > > + if (!pmd_present(pmd)) {
    > > > > + swp_entry_t entry = pmd_to_swp_entry(pmd);
    > > > > +
    > > > > + if (is_migration_entry(entry)) {
    > > >
    > > > I think you should check thp_migration_supported() here, since PMD migration is only enabled in x86_64 systems.
    > > > Other architectures should treat PMD migration entries as bad.
    > >
    > > How can we have a migration pmd entry when the migration is not
    > > supported?
    >
    > Not sure i follow here, migration can happen anywhere (assuming
    > that something like compaction is active or numa or ...). So this
    > code can face pmd migration entry on architecture that support
    > it. What is missing here is thp_migration_supported() call to
    > protect the is_migration_entry() to avoid false positive on arch
    > which do not support thp migration.

    I mean that architectures which do not support THP migration shouldn't
    ever see any migration entry. So is_migration_entry should be always
    false. Or do I miss something?
    --
    Michal Hocko
    SUSE Labs

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