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

["Zhang Haoyu" <zhanghy@sangfor.com>]

>> 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?
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