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    Subject[PATCH v3 6/6] mm: document semantics of ZONE_MOVABLE
    Date
    Let's document what ZONE_MOVABLE means, how it's used, and which special
    cases we have regarding unmovable pages (memory offlining vs. migration /
    allocations).

    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Michael S. Tsirkin <mst@redhat.com>
    Cc: Mike Kravetz <mike.kravetz@oracle.com>
    Cc: Mike Rapoport <rppt@kernel.org>
    Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
    Cc: Baoquan He <bhe@redhat.com>
    Signed-off-by: David Hildenbrand <david@redhat.com>
    ---
    include/linux/mmzone.h | 34 ++++++++++++++++++++++++++++++++++
    1 file changed, 34 insertions(+)

    diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
    index f6f884970511d..600d449e7d9e9 100644
    --- a/include/linux/mmzone.h
    +++ b/include/linux/mmzone.h
    @@ -372,6 +372,40 @@ enum zone_type {
    */
    ZONE_HIGHMEM,
    #endif
    + /*
    + * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it *primarily*
    + * only contains movable pages. Main use cases are to make memory
    + * offlining more likely to succeed, and to locally limit unmovable
    + * allocations - e.g., to increase the number of THP/huge pages.
    + * Notable special cases are:
    + *
    + * 1. Pinned pages: (Long-term) pinning of movable pages might
    + * essentially turn such pages unmovable. Memory offlining might
    + * retry a long time.
    + * 2. memblock allocations: kernelcore/movablecore setups might create
    + * situations where ZONE_MOVABLE contains unmovable allocations
    + * after boot. Memory offlining and allocations fail early.
    + * 3. Memory holes: Such pages cannot be allocated. Applies only to
    + * boot memory, not hotplugged memory. Memory offlining and
    + * allocations fail early.
    + * 4. PG_hwpoison pages: While poisoned pages can be skipped during
    + * memory offlining, such pages cannot be allocated.
    + * 5. Unmovable PG_offline pages: In paravirtualized environments,
    + * hotplugged memory blocks might only partially be managed by the
    + * buddy (e.g., via XEN-balloon, Hyper-V balloon, virtio-mem). The
    + * parts not manged by the buddy are unmovable PG_offline pages. In
    + * some cases (virtio-mem), such pages can be skipped during
    + * memory offlining, however, cannot be moved/allocated. These
    + * techniques might use alloc_contig_range() to hide previously
    + * exposed pages from the buddy again (e.g., to implement some sort
    + * of memory unplug in virtio-mem).
    + *
    + * In general, no unmovable allocations that degrade memory offlining
    + * should end up in ZONE_MOVABLE. Allocators (like alloc_contig_range())
    + * have to expect that migrating pages in ZONE_MOVABLE can fail (even
    + * if has_unmovable_pages() states that there are no unmovable pages,
    + * there can be false negatives).
    + */
    ZONE_MOVABLE,
    #ifdef CONFIG_ZONE_DEVICE
    ZONE_DEVICE,
    --
    2.26.2
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