mm: document semantics of ZONE_MOVABLE

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).
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Qian Cai <cai@lca.pw>
Link: http://lkml.kernel.org/r/20200816125333.7434-7-david@redhat.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -396,6 +396,41 @@ enum zone_type {
 	 */
 	ZONE_HIGHMEM,
 #endif
+	/*
+	 * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it contains
+	 * movable pages with few exceptional cases described below. Main use
+	 * cases for ZONE_MOVABLE are to make memory offlining/unplug 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: kernelcore/movablecore setups might create very rare
+	 *    situations where ZONE_MOVABLE contains memory holes after boot,
+	 *    for example, if we have sections that are only partially
+	 *    populated. 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,