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

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

[Kevin Wolf]

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
allocations.

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 <kwolf@redhat.com>
---
 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);
 retry:
     for(i = 0; i < nb_clusters; i++) {
-- 
1.8.3.1

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

[Max Reitz]

On 06.05.2015 13:29, Kevin Wolf wrote:
> 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
> allocations.
>
> 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 <kwolf@redhat.com>
> ---
>   block/qcow2-refcount.c | 5 +++++
>   1 file changed, 5 insertions(+)

Reviewed-by: Max Reitz <mreitz@redhat.com>

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

[Kevin Wolf]

Am 06.05.2015 um 13:29 hat Kevin Wolf geschrieben:
> 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
> allocations.
> 
> 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 <kwolf@redhat.com>

Cc: qemu-stable@nongnu.org

Applied to my block branch.

Kevin