migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination

Currently, when someone (i.e., the VM) accesses discarded parts inside a
RAMBlock with a RamDiscardManager managing the corresponding mapped memory
region, postcopy will request migration of the corresponding page from the
source. The source, however, will never answer, because it refuses to
migrate such pages with undefined content ("logically unplugged"): the
pages are never dirty, and get_queued_page() will consequently skip
processing these postcopy requests.

Especially reading discarded ("logically unplugged") ranges is supposed to
work in some setups (for example with current virtio-mem), although it
barely ever happens: still, not placing a page would currently stall the
VM, as it cannot make forward progress.

Let's check the state via the RamDiscardManager (the state e.g.,
of virtio-mem is migrated during precopy) and avoid sending a request
that will never get answered. Place a fresh zero page instead to keep
the VM working. This is the same behavior that would happen
automatically without userfaultfd being active, when accessing virtual
memory regions without populated pages -- "populate on demand".

For now, there are valid cases (as documented in the virtio-mem spec) where
a VM might read discarded memory; in the future, we will disallow that.
Then, we might want to handle that case differently, e.g., warning the
user that the VM seems to be mis-behaving.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/migration/ram.c b/migration/ram.c
index e8c06f207c..4f629de7d0 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -912,6 +912,27 @@ static uint64_t ramblock_dirty_bitmap_clear_discarded_pages(RAMBlock *rb)
     return cleared_bits;
 }
 
+/*
+ * Check if a host-page aligned page falls into a discarded range as managed by
+ * a RamDiscardManager responsible for the mapped memory region of the RAMBlock.
+ *
+ * Note: The result is only stable while migrating (precopy/postcopy).
+ */
+bool ramblock_page_is_discarded(RAMBlock *rb, ram_addr_t start)
+{
+    if (rb->mr && memory_region_has_ram_discard_manager(rb->mr)) {
+        RamDiscardManager *rdm = memory_region_get_ram_discard_manager(rb->mr);
+        MemoryRegionSection section = {
+            .mr = rb->mr,
+            .offset_within_region = start,
+            .size = int128_make64(qemu_ram_pagesize(rb)),
+        };
+
+        return !ram_discard_manager_is_populated(rdm, &section);
+    }
+    return false;
+}
+
 /* Called with RCU critical section */
 static void ramblock_sync_dirty_bitmap(RAMState *rs, RAMBlock *rb)
 {