Re: [Qemu-devel] [PATCH] snapshot: use local variable to bdrv_pwrite_syncL1 table

[Max Reitz <mreitz@redhat.com>]

On 2014-10-21 at 12:49, Zhang Haoyu wrote:
>>> Use local variable to bdrv_pwrite_sync L1 table,
>>> needless to make conversion of cached L1 table between
>>> big-endian and host style.
>>>
>>> Signed-off-by: Zhang Haoyu <zhanghy@sangfor.com>
>>> ---
>>>    block/qcow2-refcount.c | 22 +++++++---------------
>>>    1 file changed, 7 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>>> index 2bcaaf9..8b318e8 100644
>>> --- a/block/qcow2-refcount.c
>>> +++ b/block/qcow2-refcount.c
>>> @@ -881,7 +881,6 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>    {
>>>        BDRVQcowState *s = bs->opaque;
>>>        uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2;
>>> -    bool l1_allocated = false;
>>>        int64_t old_offset, old_l2_offset;
>>>        int i, j, l1_modified = 0, nb_csectors, refcount;
>>>        int ret;
>>> @@ -889,6 +888,11 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>        l2_table = NULL;
>>>        l1_table = NULL;
>> Please remove this assignment; thanks to this hunk we don't need it anymore.
> OK.
>>>        l1_size2 = l1_size * sizeof(uint64_t);
>>> +    l1_table = g_try_malloc0(align_offset(l1_size2, 512));
>> I wanted to propose using qemu_try_blockalign(), but since it'd require
>> a memset() afterwards, it gets rather ugly.
>>
>> Could you at least replace 512 by BDRV_SECTOR_SIZE, and maybe even
>> align_offset() by ROUND_UP()? We should probably do the latter in all of
>> the qcow2 code, though, I think it's just there because it has been
>> around since before there was a ROUND_UP()...
>>
> Good, I will replace 512 with BDRV_SECTOR_SIZE, and replace align_offset with ROUND_UP.
>>> +    if (l1_size2 && l1_table == NULL) {
>>> +        ret = -ENOMEM;
>>> +        goto fail;
>>> +    }
>>>    
>>>        s->cache_discards = true;
>>>    
>>> @@ -896,13 +900,6 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>         * l1_table_offset when it is the current s->l1_table_offset! Be careful
>>>         * when changing this! */
>>>        if (l1_table_offset != s->l1_table_offset) {
>>> -        l1_table = g_try_malloc0(align_offset(l1_size2, 512));
>>> -        if (l1_size2 && l1_table == NULL) {
>>> -            ret = -ENOMEM;
>>> -            goto fail;
>>> -        }
>>> -        l1_allocated = true;
>>> -
>>>            ret = bdrv_pread(bs->file, l1_table_offset, l1_table, l1_size2);
>>>            if (ret < 0) {
>>>                goto fail;
>>> @@ -912,8 +909,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>                be64_to_cpus(&l1_table[i]);
>>>        } else {
>>>            assert(l1_size == s->l1_size);
>>> -        l1_table = s->l1_table;
>>> -        l1_allocated = false;
>>> +        memcpy(l1_table, s->l1_table, l1_size2);
>>>        }
>>>    
>>>        for(i = 0; i < l1_size; i++) {
>>> @@ -1055,12 +1051,8 @@ fail:
>> I don't think it will change a lot, but could you wrap the
>> "s->cache_discards = false; qcow2_process_discards(bs, ret);" in an "if
>> (s->cache_discards)"? You have introduced a case where s->cache_discards
>> was still false, so we don't need to call qcow2_process_discards() then
>> (which will hopefully return immediately, but well...).
> s->cache_discards's initial value is true in qcow2_update_snapshot_refcount(),
> where s->cache_discards is set to false?

It is? I only see it set to true after the "if (l1_size2 && l1_table == 
NULL)" conditional block.

Well, okay, I don't know about the callers of qcow2_process_discards(), 
so they may have set s->cache_discards to true and then expect this 
function to always call qcow2_process_discards() and set 
s->cache_discards to false. Okay, then let's just keep it as it is.

Max

> Or you means s->cache_discards should be set to false
> after g_try_malloc0(align_offset(l1_size2, 512)) failed.
>
>>>            }
>>>    
>>>            ret = bdrv_pwrite_sync(bs->file, l1_table_offset, l1_table, l1_size2);
>>> -
>>> -        for (i = 0; i < l1_size; i++) {
>>> -            be64_to_cpus(&l1_table[i]);
>>> -        }
>>>        }
>>> -    if (l1_allocated)
>>> +    if (l1_table)
>>>            g_free(l1_table);
>> Just drop the condition. g_free(l1_table); is enough.
>>
> OK.
>>>        return ret;
>>>    }
>> The change itself is good, it just needs some polishing.
>>
>> Max