diff mbox

[v2,3/7] qcow2: Make perform_cow() call do_perform_cow() twice

Message ID c49daf8afe80eade79ac53d305b83eb24b264338.1496844254.git.berto@igalia.com
State New
Headers show

Commit Message

Alberto Garcia June 7, 2017, 2:08 p.m. UTC 
Instead of calling perform_cow() twice with a different COW region
each time, call it just once and make perform_cow() handle both
regions.

This patch simply moves code around. The next one will do the actual
reordering of the COW operations.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 block/qcow2-cluster.c | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)

Comments

Eric Blake June 7, 2017, 9:43 p.m. UTC | #1 
On 06/07/2017 09:08 AM, Alberto Garcia wrote:
> Instead of calling perform_cow() twice with a different COW region
> each time, call it just once and make perform_cow() handle both
> regions.
> 
> This patch simply moves code around. The next one will do the actual
> reordering of the COW operations.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c | 38 +++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 15 deletions(-)

>      qemu_co_mutex_unlock(&s->lock);
> -    ret = do_perform_cow(bs, m->offset, m->alloc_offset, r->offset, r->nb_bytes);
> +    ret = do_perform_cow(bs, m->offset, m->alloc_offset,
> +                         start->offset, start->nb_bytes);
> +    if (ret < 0) {
> +        goto fail;
> +    }
> +
> +    ret = do_perform_cow(bs, m->offset, m->alloc_offset,
> +                         end->offset, end->nb_bytes);
> +
> +fail:

Since you reach this label even on success, it feels a bit awkward. I
probably would have avoided the goto and used fewer lines by refactoring
the logic as:

ret = do_perform_cow(..., start->...);
if (ret >= 0) {
    ret = do_perform_cow(..., end->...);
}

But that doesn't affect correctness, so whether or not you redo the
logic in the shorter fashion:

Reviewed-by: Eric Blake <eblake@redhat.com>
Alberto Garcia June 8, 2017, 7:09 a.m. UTC | #2 
On Wed 07 Jun 2017 11:43:34 PM CEST, Eric Blake wrote:
>>  block/qcow2-cluster.c | 38 +++++++++++++++++++++++---------------
>>  1 file changed, 23 insertions(+), 15 deletions(-)
>
>>      qemu_co_mutex_unlock(&s->lock);
>> -    ret = do_perform_cow(bs, m->offset, m->alloc_offset, r->offset, r->nb_bytes);
>> +    ret = do_perform_cow(bs, m->offset, m->alloc_offset,
>> +                         start->offset, start->nb_bytes);
>> +    if (ret < 0) {
>> +        goto fail;
>> +    }
>> +
>> +    ret = do_perform_cow(bs, m->offset, m->alloc_offset,
>> +                         end->offset, end->nb_bytes);
>> +
>> +fail:
>
> Since you reach this label even on success, it feels a bit awkward. I
> probably would have avoided the goto and used fewer lines by
> refactoring the logic as:
>
> ret = do_perform_cow(..., start->...);
> if (ret >= 0) {
>     ret = do_perform_cow(..., end->...);
> }

I actually wrote this code without any gotos the way you mention, and it
works fine in this patch, but as I was making more changes I ended up
with a quite large piece of code that needed to add "if (!ret)" to
almost every if statement, so I didn't quite like the final result.

I'm open to reconsider it, but take a look first at the code after the
last patch and you'll see that it would not be as neat as it appears
now.

Berto
diff mbox

Patch

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index a86c5a75a9..4c03639a72 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -414,6 +414,10 @@  static int coroutine_fn do_perform_cow(BlockDriverState *bs,
     struct iovec iov;
     int ret;
 
+    if (bytes == 0) {
+        return 0;
+    }
+
     iov.iov_len = bytes;
     iov.iov_base = qemu_try_blockalign(bs, iov.iov_len);
     if (iov.iov_base == NULL) {
@@ -751,31 +755,40 @@  uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
     return cluster_offset;
 }
 
-static int perform_cow(BlockDriverState *bs, QCowL2Meta *m, Qcow2COWRegion *r)
+static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
 {
     BDRVQcow2State *s = bs->opaque;
+    Qcow2COWRegion *start = &m->cow_start;
+    Qcow2COWRegion *end = &m->cow_end;
     int ret;
 
-    if (r->nb_bytes == 0) {
+    if (start->nb_bytes == 0 && end->nb_bytes == 0) {
         return 0;
     }
 
     qemu_co_mutex_unlock(&s->lock);
-    ret = do_perform_cow(bs, m->offset, m->alloc_offset, r->offset, r->nb_bytes);
+    ret = do_perform_cow(bs, m->offset, m->alloc_offset,
+                         start->offset, start->nb_bytes);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = do_perform_cow(bs, m->offset, m->alloc_offset,
+                         end->offset, end->nb_bytes);
+
+fail:
     qemu_co_mutex_lock(&s->lock);
 
-    if (ret < 0) {
-        return ret;
-    }
-
     /*
      * Before we update the L2 table to actually point to the new cluster, we
      * need to be sure that the refcounts have been increased and COW was
      * handled.
      */
-    qcow2_cache_depends_on_flush(s->l2_table_cache);
+    if (ret == 0) {
+        qcow2_cache_depends_on_flush(s->l2_table_cache);
+    }
 
-    return 0;
+    return ret;
 }
 
 int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
@@ -795,12 +808,7 @@  int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     }
 
     /* copy content of unmodified sectors */
-    ret = perform_cow(bs, m, &m->cow_start);
-    if (ret < 0) {
-        goto err;
-    }
-
-    ret = perform_cow(bs, m, &m->cow_end);
+    ret = perform_cow(bs, m);
     if (ret < 0) {
         goto err;
     }