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    SubjectRe: [RFC PATCH] mm,memory_hotplug: Unlock 1GB-hugetlb on x86_64
    On Thu 28-02-19 10:41:08, Oscar Salvador wrote:
    > On Thu, Feb 28, 2019 at 10:21:54AM +0100, Michal Hocko wrote:
    > > On Thu 21-02-19 10:42:12, Oscar Salvador wrote:
    > > [...]
    > > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
    > > > index d5f7afda67db..04f6695b648c 100644
    > > > --- a/mm/memory_hotplug.c
    > > > +++ b/mm/memory_hotplug.c
    > > > @@ -1337,8 +1337,7 @@ static unsigned long scan_movable_pages(unsigned long start, unsigned long end)
    > > > if (!PageHuge(page))
    > > > continue;
    > > > head = compound_head(page);
    > > > - if (hugepage_migration_supported(page_hstate(head)) &&
    > > > - page_huge_active(head))
    > > > + if (page_huge_active(head))
    > > > return pfn;
    > > > skip = (1 << compound_order(head)) - (page - head);
    > > > pfn += skip - 1;
    > >
    > > Is this part correct? Say we have a gigantic page which is migrateable.
    > > Now scan_movable_pages would skip it and we will not migrate it, no?
    >
    > All non-migrateable hugepages should have been caught in has_unmovable_pages:
    >
    > <--
    > if (PageHuge(page)) {
    > struct page *head = compound_head(page);
    > unsigned int skip_pages;
    >
    > if (!hugepage_migration_supported(page_hstate(head)))
    > goto unmovable;
    > -->
    >
    > So, there is no need to check again for migrateability here, as it is something
    > that does not change.
    > To put it in another way, all huge pages found in scan_movable_pages() should be
    > migrateable.
    > In scan_movable_pages() we just need to check whether the hugepage, gigantic or not, is
    > in use (aka active) to migrate it.

    You seemed to miss my point or I am wrong here. If scan_movable_pages
    skips over a hugetlb page then there is nothing to migrate it and it
    will stay in the pfn range and the range will not become idle.

    --
    Michal Hocko
    SUSE Labs

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