Postcopy: Mark nohugepage before discard

Prior to servicing userfault requests we must ensure we've not got
huge pages in the area that might include non-transferred memory,
since a hugepage could incorrectly mark the whole huge page as present.

We mark the area as non-huge page (nhp) just before we perform
discards; the discard code now tells us to discard any areas
that haven't been sent (as well as any that are redirtied);
any already formed transparent-huge-pages get fragmented
by this discard process if they cotnain any discards.

Transparent huge pages that have been entirely transferred
and don't contain any discards are not broken by this mechanism;
they stay as huge pages.

By starting postcopy after a full precopy pass, many of the pages
then stay as huge pages; this is important for maintaining performance
after the end of the migration.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/migration/savevm.c b/migration/savevm.c
index eb32199888..0596f7bc61 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1316,7 +1316,7 @@ static int loadvm_postcopy_ram_handle_discard(MigrationIncomingState *mis,
     switch (ps) {
     case POSTCOPY_INCOMING_ADVISE:
         /* 1st discard */
-        tmp = 0; /* TODO: later patch postcopy_ram_prepare_discard(mis); */
+        tmp = postcopy_ram_prepare_discard(mis);
         if (tmp) {
             return tmp;
         }
@@ -1448,6 +1448,13 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
         error_report("CMD_POSTCOPY_LISTEN in wrong postcopy state (%d)", ps);
         return -1;
     }
+    if (ps == POSTCOPY_INCOMING_ADVISE) {
+        /*
+         * A rare case, we entered listen without having to do any discards,
+         * so do the setup that's normally done at the time of the 1st discard.
+         */
+        postcopy_ram_prepare_discard(mis);
+    }
 
     /*
      * Sensitise RAM - can now generate requests for blocks that don't exist