Re: [Qemu-block] [PATCH v2 3/7] qcow2: Make perform_cow() call do_perform_cow() twice

[Alberto Garcia]

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(>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

Re: [Qemu-block] [PATCH v2 3/7] qcow2: Make perform_cow() call do_perform_cow() twice

[Eric Blake]

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 
> ---
>  block/qcow2-cluster.c | 38 +++---
>  1 file changed, 23 insertions(+), 15 deletions(-)

>  qemu_co_mutex_unlock(>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 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



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[Qemu-block] [PATCH v2 3/7] qcow2: Make perform_cow() call do_perform_cow() twice

[Alberto Garcia]

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 
---
 block/qcow2-cluster.c | 38 +++---
 1 file changed, 23 insertions(+), 15 deletions(-)

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 = >cow_start;
+Qcow2COWRegion *end = >cow_end;
 int ret;
 
-if (r->nb_bytes == 0) {
+if (start->nb_bytes == 0 && end->nb_bytes == 0) {
 return 0;
 }
 
 qemu_co_mutex_unlock(>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(>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, >cow_start);
-if (ret < 0) {
-goto err;
-}
-
-ret = perform_cow(bs, m, >cow_end);
+ret = perform_cow(bs, m);
 if (ret < 0) {
 goto err;
 }
-- 
2.11.0