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[Qemu-devel] [PATCH] qcow2: Flush pending discards before allocating clu

From: Kevin Wolf
Subject: [Qemu-devel] [PATCH] qcow2: Flush pending discards before allocating cluster
Date: Wed, 6 May 2015 13:29:34 +0200

Before a freed cluster can be reused, pending discards for this cluster
must be processed.

The original assumption was that this was not a problem because discards
are only cached during discard/write zeroes operations, which are
synchronous so that no concurrent write requests can cause cluster

However, the discard/write zeroes operation itself can allocate a new L2
table (and it has to in order to put zero flags there), so make sure we
can cope with the situation.

This fixes https://bugs.launchpad.net/bugs/1349972.

Signed-off-by: Kevin Wolf <address@hidden>
 block/qcow2-refcount.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index f47260b..83467c3 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -833,6 +833,11 @@ static int64_t alloc_clusters_noref(BlockDriverState *bs, 
uint64_t size)
     uint64_t i, nb_clusters, refcount;
     int ret;
+    /* We can't allocate clusters if they may still be queued for discard. */
+    if (s->cache_discards) {
+        qcow2_process_discards(bs, 0);
+    }
     nb_clusters = size_to_clusters(s, size);
     for(i = 0; i < nb_clusters; i++) {

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