[PATCH v2 6/6] mm: document semantics of ZONE_MOVABLE

[David Hildenbrand <david@redhat.com>]

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..b8c49b2aff684 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:
+	 *
+	 * 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/allcoated. 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