[PATCH] migration/multifd: Document two places for mapped-ram

[peterx@redhat.com]

From: Peter Xu <peterx@redhat.com>

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

Signed-off-by: Peter Xu <peterx@redhat.com>
---
Based-on: <20240229153017.2221-1-farosas@suse.de>
---
 migration/multifd.c | 12 ++++++++++++
 migration/ram.c     |  8 +++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index b4e5a9dfcc..2942395ce2 100644
--- a/migration/multifd.c
+++ b/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 explicitly close() on the channel here is normally not
+         * required, but can be helpful for "file:" iochannels, where it
+         * will include an 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;
diff --git a/migration/ram.c b/migration/ram.c
index 1f1b5297cf..c79e3de521 100644
--- a/migration/ram.c
+++ b/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();
             }
-- 
2.44.0