[PATCH v3 6/6] mm: document semantics of ZONE_MOVABLE

David Hildenbrand
2020-08-04T07:24:50Z

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