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>

migration/postcopy-ram.c

@@ -671,6 +671,29 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd,
     return ret;
 }
 
+static int postcopy_request_page(MigrationIncomingState *mis, RAMBlock *rb,
+                                 ram_addr_t start, uint64_t haddr)
+{
+    void *aligned = (void *)(uintptr_t)ROUND_DOWN(haddr, qemu_ram_pagesize(rb));
+
+    /*
+     * Discarded pages (via RamDiscardManager) are never migrated. On unlikely
+     * access, place a zeropage, which will also set the relevant bits in the
+     * recv_bitmap accordingly, so we won't try placing a zeropage twice.
+     *
+     * Checking a single bit is sufficient to handle pagesize > TPS as either
+     * all relevant bits are set or not.
+     */
+    assert(QEMU_IS_ALIGNED(start, qemu_ram_pagesize(rb)));
+    if (ramblock_page_is_discarded(rb, start)) {
+        bool received = ramblock_recv_bitmap_test_byte_offset(rb, start);
+
+        return received ? 0 : postcopy_place_page_zero(mis, aligned, rb);
+    }
+
+    return migrate_send_rp_req_pages(mis, rb, start, haddr);
+}
+
 /*
  * Callback from shared fault handlers to ask for a page,
  * the page must be specified by a RAMBlock and an offset in that rb
@@ -690,7 +713,7 @@ int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
                                         qemu_ram_get_idstr(rb), rb_offset);
         return postcopy_wake_shared(pcfd, client_addr, rb);
     }
-    migrate_send_rp_req_pages(mis, rb, aligned_rbo, client_addr);
+    postcopy_request_page(mis, rb, aligned_rbo, client_addr);
     return 0;
 }
 
@@ -984,7 +1007,7 @@ retry:
              * Send the request to the source - we want to request one
              * of our host page sizes (which is >= TPS)
              */
-            ret = migrate_send_rp_req_pages(mis, rb, rb_offset,
+            ret = postcopy_request_page(mis, rb, rb_offset,
                                         msg.arg.pagefault.address);
             if (ret) {
                 /* May be network failure, try to wait for recovery */
@@ -993,7 +1016,7 @@ retry:
                     goto retry;
                 } else {
                     /* This is a unavoidable fault */
-                    error_report("%s: migrate_send_rp_req_pages() get %d",
+                    error_report("%s: postcopy_request_page() get %d",
                                  __func__, ret);
                     break;
                 }

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)
 {

migration/ram.h

@@ -72,6 +72,7 @@ void ramblock_recv_bitmap_set_range(RAMBlock *rb, void *host_addr, size_t nr);
 int64_t ramblock_recv_bitmap_send(QEMUFile *file,
                                   const char *block_name);
 int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *rb);
+bool ramblock_page_is_discarded(RAMBlock *rb, ram_addr_t start);
 
 /* ram cache */
 int colo_init_ram_cache(void);