[Qemu-devel] [PATCH 1/2] Postcopy: Force allocation of all-zero precopy pages

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

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

When an all-zero page is received during the precopy
phase of a postcopy-enabled migration we must force
allocation otherwise accesses to the page will still
get blocked by userfault.

Symptom:
  a) If the page is accessed by a device during device-load
    then we get a deadlock as the source finishes sending
    all its pages but the destination device-load is still
    paused and so doesn't clean up.

  b) If the page is accessed later, then the thread will stay
    paused until the end of migration rather than carrying on
    running, until we release userfault at the end.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 include/migration/migration.h |  3 ++-
 migration/ram.c               | 12 ++++++++----
 migration/rdma.c              |  2 +-
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/migration/migration.h b/include/migration/migration.h
index ba1a16cbc1..b47904033c 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -265,7 +265,8 @@ uint64_t xbzrle_mig_pages_overflow(void);
 uint64_t xbzrle_mig_pages_cache_miss(void);
 double xbzrle_mig_cache_miss_rate(void);
 
-void ram_handle_compressed(void *host, uint8_t ch, uint64_t size);
+void ram_handle_compressed(void *host, uint8_t ch, uint64_t size,
+                           bool always_write);
 void ram_debug_dump_bitmap(unsigned long *todump, bool expected);
 /* For outgoing discard bitmap */
 int ram_postcopy_send_discard_bitmap(MigrationState *ms);
diff --git a/migration/ram.c b/migration/ram.c
index f48664ec62..b4ed41c725 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2274,10 +2274,12 @@ static inline void *host_from_ram_block_offset(RAMBlock *block,
  * @host: host address for the zero page
  * @ch: what the page is filled from.  We only support zero
  * @size: size of the zero page
+ * @always_write: Always perform the memset even if it's zero
  */
-void ram_handle_compressed(void *host, uint8_t ch, uint64_t size)
+void ram_handle_compressed(void *host, uint8_t ch, uint64_t size,
+                           bool always_write)
 {
-    if (ch != 0 || !is_zero_range(host, size)) {
+    if (ch != 0 || always_write || !is_zero_range(host, size)) {
         memset(host, ch, size);
     }
 }
@@ -2514,7 +2516,8 @@ static int ram_load_postcopy(QEMUFile *f)
         switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
         case RAM_SAVE_FLAG_COMPRESS:
             ch = qemu_get_byte(f);
-            memset(page_buffer, ch, TARGET_PAGE_SIZE);
+            ram_handle_compressed(page_buffer, ch, TARGET_PAGE_SIZE,
+                                  true);
             if (ch) {
                 all_zero = false;
             }
@@ -2664,7 +2667,8 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
 
         case RAM_SAVE_FLAG_COMPRESS:
             ch = qemu_get_byte(f);
-            ram_handle_compressed(host, ch, TARGET_PAGE_SIZE);
+            ram_handle_compressed(host, ch, TARGET_PAGE_SIZE,
+                                  postcopy_advised);
             break;
 
         case RAM_SAVE_FLAG_PAGE:
diff --git a/migration/rdma.c b/migration/rdma.c
index fe0a4b5a83..07a9bd75d8 100644
--- a/migration/rdma.c
+++ b/migration/rdma.c
@@ -3164,7 +3164,7 @@ static int qemu_rdma_registration_handle(QEMUFile *f, void *opaque)
             host_addr = block->local_host_addr +
                             (comp->offset - block->offset);
 
-            ram_handle_compressed(host_addr, comp->value, comp->length);
+            ram_handle_compressed(host_addr, comp->value, comp->length, false);
             break;
 
         case RDMA_CONTROL_REGISTER_FINISHED:
-- 
2.12.2