Re: [Qemu-devel] [PATCH v3 18/39] qcow2: Refactor get_cluster_table()

[Max Reitz <mreitz@redhat.com>]

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On 2018-02-01 11:40, Alberto Garcia wrote:
> On Wed 31 Jan 2018 09:11:48 PM CET, Max Reitz wrote:
>> On 2018-01-26 15:59, Alberto Garcia wrote:
>>> After the previous patch we're now always using l2_load() in
>>> get_cluster_table() regardless of whether a new L2 table has to be
>>> allocated or not.
>>>
>>> This patch refactors that part of the code to use one single l2_load()
>>> call.
>>>
>>> Signed-off-by: Alberto Garcia <berto@igalia.com>
>>> ---
>>>  block/qcow2-cluster.c | 21 +++++++--------------
>>>  1 file changed, 7 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>>> index 2a53c1dc5f..0c0cab85e8 100644
>>> --- a/block/qcow2-cluster.c
>>> +++ b/block/qcow2-cluster.c
>>> @@ -695,15 +695,7 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
>>>          return -EIO;
>>>      }
>>>  
>>> -    /* seek the l2 table of the given l2 offset */
>>> -
>>> -    if (s->l1_table[l1_index] & QCOW_OFLAG_COPIED) {
>>> -        /* load the l2 table in memory */
>>> -        ret = l2_load(bs, offset, l2_offset, &l2_table);
>>> -        if (ret < 0) {
>>> -            return ret;
>>> -        }
>>> -    } else {
>>> +    if (!(s->l1_table[l1_index] & QCOW_OFLAG_COPIED)) {
>>>          /* First allocate a new L2 table (and do COW if needed) */
>>>          ret = l2_allocate(bs, l1_index);
>>>          if (ret < 0) {
>>> @@ -719,11 +711,12 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
>>>          /* Get the offset of the newly-allocated l2 table */
>>>          l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
>>>          assert(offset_into_cluster(s, l2_offset) == 0);
>>> -        /* Load the l2 table in memory */
>>> -        ret = l2_load(bs, offset, l2_offset, &l2_table);
>>> -        if (ret < 0) {
>>> -            return ret;
>>> -        }
>>> +    }
>>> +
>>> +    /* load the l2 table in memory */
>>> +    ret = l2_load(bs, offset, l2_offset, &l2_table);
>>> +    if (ret < 0) {
>>> +        return ret;
>>>      }
>>
>> I'd pull the l2_offset assignment (and alignment check) down below the
>> QCOW_OFLAG_COPIED check, saving us another two lines (!!1!) which we
>> could then spend on an
>>
>> assert(s->l1_table[l1_index] & QCOW_OFLAG_COPIED);
> 
> I'm not sure if I'm following you. We need to set l2_offset before and
> after allocating a new L2 table.
> 
> Before, because we need the offset of the old table (if there was one)
> in order to decrease its refcount.
> 
> And after, because we need the offset of the new table in order to load
> it.

Ah, right, yeah... Well, too bad.  Then I probably can't ask for the
assert() that badly.

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


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