migration/multifd: Document two places for mapped-ram

Add two documentations for mapped-ram migration on two spots that may not
be extremely clear.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240301091524.39900-1-peterx@redhat.com
Cc: Prasad Pandit <ppandit@redhat.com>
[peterx: fix two English errors per Prasad]
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/multifd.c

@@ -709,6 +709,18 @@ static bool multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
 {
     if (p->c) {
         migration_ioc_unregister_yank(p->c);
+        /*
+         * An explicit close() on the channel here is normally not
+         * required, but can be helpful for "file:" iochannels, where it
+         * will include fdatasync() to make sure the data is flushed to the
+         * disk backend.
+         *
+         * The object_unref() cannot guarantee that because: (1) finalize()
+         * of the iochannel is only triggered on the last reference, and
+         * it's not guaranteed that we always hold the last refcount when
+         * reaching here, and, (2) even if finalize() is invoked, it only
+         * does a close(fd) without data flush.
+         */
         qio_channel_close(p->c, &error_abort);
         object_unref(OBJECT(p->c));
         p->c = NULL;

migration/ram.c

@@ -4258,7 +4258,13 @@ static int ram_load_precopy(QEMUFile *f)
         switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
         case RAM_SAVE_FLAG_MEM_SIZE:
             ret = parse_ramblocks(f, addr);
-
+            /*
+             * For mapped-ram migration (to a file) using multifd, we sync
+             * once and for all here to make sure all tasks we queued to
+             * multifd threads are completed, so that all the ramblocks
+             * (including all the guest memory pages within) are fully
+             * loaded after this sync returns.
+             */
             if (migrate_mapped_ram()) {
                 multifd_recv_sync_main();
             }