virtio-mem: retry fake-offlining via alloc_contig_range() on ZONE_MOVABLE

ZONE_MOVABLE is supposed to give some guarantees, yet,
alloc_contig_range() isn't prepared to properly deal with some racy
cases properly (e.g., temporary page pinning when exiting processed, PCP).

Retry 5 times for now. There is certainly room for improvement in the
future.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-11-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index c240552..2f1ce4d 100644 (file)
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c
@@ -784,19 +784,34 @@ static void virtio_mem_fake_online(unsigned long pfn, unsigned long nr_pages)
  */
 static int virtio_mem_fake_offline(unsigned long pfn, unsigned long nr_pages)
 {
-       int rc;
+       const bool is_movable = zone_idx(page_zone(pfn_to_page(pfn))) ==
+                               ZONE_MOVABLE;
+       int rc, retry_count;
 
-       rc = alloc_contig_range(pfn, pfn + nr_pages, MIGRATE_MOVABLE,
-                               GFP_KERNEL);
-       if (rc == -ENOMEM)
-               /* whoops, out of memory */
-               return rc;
-       if (rc)
-               return -EBUSY;
+       /*
+        * TODO: We want an alloc_contig_range() mode that tries to allocate
+        * harder (e.g., dealing with temporarily pinned pages, PCP), especially
+        * with ZONE_MOVABLE. So for now, retry a couple of times with
+        * ZONE_MOVABLE before giving up - because that zone is supposed to give
+        * some guarantees.
+        */
+       for (retry_count = 0; retry_count < 5; retry_count++) {
+               rc = alloc_contig_range(pfn, pfn + nr_pages, MIGRATE_MOVABLE,
+                                       GFP_KERNEL);
+               if (rc == -ENOMEM)
+                       /* whoops, out of memory */
+                       return rc;
+               else if (rc && !is_movable)
+                       break;
+               else if (rc)
+                       continue;
 
-       virtio_mem_set_fake_offline(pfn, nr_pages, true);
-       adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
-       return 0;
+               virtio_mem_set_fake_offline(pfn, nr_pages, true);
+               adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
+               return 0;
+       }
+
+       return -EBUSY;
 }
 
 /*