[Qemu-devel] [PATCH v6 47/47] Inhibit ballooning during postcopy

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Postcopy detects accesses to pages that haven't been transferred yet
using userfaultfd, and it causes exceptions on pages that are 'not
present'.
Ballooning also causes pages to be marked as 'not present' when the
guest inflates the balloon.
Potentially a balloon could be inflated to discard pages that are
currently inflight during postcopy and that may be arriving at about
the same time.

To avoid this confusion, disable ballooning during postcopy.

When disabled we drop balloon requests from the guest.  Since ballooning
is generally initiated by the host, the management system should avoid
initiating any balloon instructions to the guest during migration,
although it's not possible to know how long it would take a guest to
process a request made prior to the start of migration.

Queueing the requests until after migration would be nice, but is
non-trivial, since the set of inflate/deflate requests have to
be compared with the state of the page to know what the final
outcome is allowed to be.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 balloon.c                  | 11 +++++++++++
 hw/virtio/virtio-balloon.c |  4 +++-
 include/sysemu/balloon.h   |  2 ++
 migration/postcopy-ram.c   |  9 +++++++++
 4 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/balloon.c b/balloon.c
index 70c00f5..0274df8 100644
--- a/balloon.c
+++ b/balloon.c
@@ -35,6 +35,17 @@
 static QEMUBalloonEvent *balloon_event_fn;
 static QEMUBalloonStatus *balloon_stat_fn;
 static void *balloon_opaque;
+static bool balloon_inhibited;
+
+bool qemu_balloon_is_inhibited(void)
+{
+    return balloon_inhibited;
+}
+
+void qemu_balloon_inhibit(bool state)
+{
+    balloon_inhibited = state;
+}
 
 static bool have_balloon(Error **errp)
 {
diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 95b0643..8bb93db 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -37,9 +37,11 @@
 static void balloon_page(void *addr, int deflate)
 {
 #if defined(__linux__)
-    if (!kvm_enabled() || kvm_has_sync_mmu())
+    if (!qemu_balloon_is_inhibited() && (!kvm_enabled() ||
+                                         kvm_has_sync_mmu())) {
         qemu_madvise(addr, TARGET_PAGE_SIZE,
                 deflate ? QEMU_MADV_WILLNEED : QEMU_MADV_DONTNEED);
+    }
 #endif
 }
 
diff --git a/include/sysemu/balloon.h b/include/sysemu/balloon.h
index 0345e01..6851d99 100644
--- a/include/sysemu/balloon.h
+++ b/include/sysemu/balloon.h
@@ -23,5 +23,7 @@ typedef void (QEMUBalloonStatus)(void *opaque, BalloonInfo *info);
 int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
 			     QEMUBalloonStatus *stat_func, void *opaque);
 void qemu_remove_balloon_handler(void *opaque);
+bool qemu_balloon_is_inhibited(void);
+void qemu_balloon_inhibit(bool state);
 
 #endif
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index ddf0841..50ce6eb 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -24,6 +24,7 @@
 #include "migration/migration.h"
 #include "migration/postcopy-ram.h"
 #include "sysemu/sysemu.h"
+#include "sysemu/balloon.h"
 #include "qemu/error-report.h"
 #include "trace.h"
 
@@ -316,6 +317,8 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
         mis->have_fault_thread = false;
     }
 
+    qemu_balloon_inhibit(false);
+
     if (enable_mlock) {
         if (os_mlock() < 0) {
             error_report("mlock: %s", strerror(errno));
@@ -514,6 +517,12 @@ int postcopy_ram_enable_notify(MigrationIncomingState *mis)
         return -1;
     }
 
+    /*
+     * Ballooning can mark pages as absent while we're postcopying
+     * that would cause false userfaults.
+     */
+    qemu_balloon_inhibit(true);
+
     trace_postcopy_ram_enable_notify();
 
     return 0;
-- 
2.1.0