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, 6 insertions, 0 deletions

diff --git a/include/migration/postcopy-ram.h b/include/migration/postcopy-ram.h
index d7c292f..b6a7491 100644
--- a/include/migration/postcopy-ram.h
+++ b/include/migration/postcopy-ram.h
@@ -41,6 +41,12 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis);
 int postcopy_ram_discard_range(MigrationIncomingState *mis, uint8_t *start,
                                size_t length);
 
+/*
+ * Userfault requires us to mark RAM as NOHUGEPAGE prior to discard
+ * however leaving it until after precopy means that most of the precopy
+ * data is still THPd
+ */
+int postcopy_ram_prepare_discard(MigrationIncomingState *mis);
 
 /*
  * Called at the start of each RAMBlock by the bitmap code.