We can already have pages that can be offlined but not allocated in ZONE_MOVABLE - PageHWPoison pages. While these pages can be skipped when offlining ("moving them to /dev/null"), we cannot move them when allocating.
virtio-mem managed memory is similar. The logical memory holes corresponding to unplug memory ranges can be skipped when offlining, however, the pages cannot be moved. Currently, virtio-mem special-cases ZONE_MOVABLE, such that: - partially plugged memory blocks it added to Linux cannot be onlined to ZONE_MOVABLE - when unplugging memory, it will never consider memory blocks that were onlined to ZONE_MOVABLE We also want to support ZONE_MOVABLE in virtio-mem for both cases. Note that virtio-mem does not blindly try to unplug random pages within its managed memory region. It always plugs memory left-to-right and tries to unplug memory right-to-left - in roughly MAX_ORDER - 1 granularity. In theory, the movable ZONE part would only shrink when unplugging memory from ZONE_MOVABLE. Let's perform the ZONE_MOVABLE optimization only for memory offlining, such that we reduce the number of false positives from has_unmovable_pages() in case of alloc_contig_range() on ZONE_MOVABLE. Note: We currently don't seem to have any user of alloc_contig_range() that actually uses ZONE_MOVABLE. This change is mostly valuable for the documentation. Cc: Andrew Morton <a...@linux-foundation.org> Cc: Michal Hocko <mho...@suse.com> Cc: Michael S. Tsirkin <m...@redhat.com> Signed-off-by: David Hildenbrand <da...@redhat.com> --- mm/page_alloc.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index bd3ebf08f09b9..45077d74d975d 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -8237,9 +8237,12 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page, /* * If the zone is movable and we have ruled out all reserved * pages then it should be reasonably safe to assume the rest - * is movable. + * is movable. As we can have some pages in the movable zone + * that are only considered movable for memory offlining (esp., + * PageHWPoison and PageOffline that will be skipped), we + * perform this optimization only for memory offlining. */ - if (zone_idx(zone) == ZONE_MOVABLE) + if ((flags & MEMORY_OFFLINE) && zone_idx(zone) == ZONE_MOVABLE) continue; /* -- 2.26.2 _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization