[Qemu-devel] [PATCH] qed: Images with backing file do not require QED_F_NEED_CHECK

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

The consistency check on open is necessary in order to fix inconsistent
table offsets left as a result of a crash mid-operation.  Images with a
backing file actually flush before updating table offsets and are
therefore guaranteed to be consistent.  Do not mark these images dirty.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block/qed.c |   24 +++++++++++++++++-------
 1 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/block/qed.c b/block/qed.c
index a46f9ef..3273448 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -977,6 +977,19 @@ static void qed_aio_write_prefill(void *opaque, int ret)
 }
 
 /**
+ * Check if the QED_F_NEED_CHECK bit should be set during allocating write
+ */
+static bool qed_should_set_need_check(BDRVQEDState *s)
+{
+    /* The flush before L2 update path ensures consistency */
+    if (s->bs->backing_hd) {
+        return false;
+    }
+
+    return !(s->header.features & QED_F_NEED_CHECK);
+}
+
+/**
  * Write new data cluster
  *
  * @acb:        Write request
@@ -1001,15 +1014,12 @@ static void qed_aio_write_alloc(QEDAIOCB *acb, size_t len)
     acb->cur_cluster = qed_alloc_clusters(s, acb->cur_nclusters);
     qemu_iovec_copy(&acb->cur_qiov, acb->qiov, acb->qiov_offset, len);
 
-    /* Write new cluster if the image is already marked dirty */
-    if (s->header.features & QED_F_NEED_CHECK) {
+    if (qed_should_set_need_check(s)) {
+        s->header.features |= QED_F_NEED_CHECK;
+        qed_write_header(s, qed_aio_write_prefill, acb);
+    } else {
         qed_aio_write_prefill(acb, 0);
-        return;
     }
-
-    /* Mark the image dirty before writing the new cluster */
-    s->header.features |= QED_F_NEED_CHECK;
-    qed_write_header(s, qed_aio_write_prefill, acb);
 }
 
 /**
-- 
1.7.2.3