Re: [Qemu-devel] [Qemu-stable] [PATCH v2] snapshot: use local variable tobdrv_pwrite_sync L1 table

[Max Reitz <mreitz@redhat.com>]

On 2014-10-21 at 15:10, Max Reitz wrote:
> On 2014-10-21 at 15:08, 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>
>>>> ---
>>>> v1 -> v2:
>>>>    - remove the superflous assignment, l1_table = NULL;
>>>>    - replace 512 with BDRV_SECTOR_SIZE, and align_offset with ROUND_UP
>>>>    - remove needless check of if (l1_table) before g_free(l1_table)
>>>>
>>>>    block/qcow2-refcount.c | 24 +++++++-----------------
>>>>    1 file changed, 7 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>>>> index 2bcaaf9..29a916a 100644
>>>> --- a/block/qcow2-refcount.c
>>>> +++ b/block/qcow2-refcount.c
>>>> @@ -881,14 +881,17 @@ 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;
>>>>           l2_table = NULL;
>>>> -    l1_table = NULL;
>>>>        l1_size2 = l1_size * sizeof(uint64_t);
>>>> +    l1_table = g_try_malloc0(ROUND_UP(l1_size2, BDRV_SECTOR_SIZE));
>>> I'm sorry, but I just now realized that we don't even need the 0 
>>> variant
>>> here. We are never accessing any element beyond l1_table[l1_size - 1],
>>> and all elements from 0 until l1_size - 1 are overwritten by
>>> bdrv_pread() or memcpy(). Therefore we can simply use
>>> qemu_try_blockalign(bs->file, l1_size2) here.
>>>
>> OK, I will replace g_try_malloc0(ROUND_UP(l1_size2, 
>> BDRV_SECTOR_SIZE)) with
>> qemu_try_blockalign(bs, ROUND_UP(l1_size2, BDRV_SECTOR_SIZE)).
>
> You can actually omit the ROUND_UP(). There are no accesses beyond 
> l1_table[l1_size - 1], if I'm not mistaken.

Oh, and it should be qemu_try_blockalign(bs->file, l1_size2), not 
qemu_try_blockalign(bs, l1_size2).

> Max
>
>>> (and we should be using qemu_{try_,}blockalign() instead of
>>> g_{try_,}malloc{0,}() whenever possible for performance reasons)
>>>
>>>> +    if (l1_size2 && l1_table == NULL) {
>>>> +        ret = -ENOMEM;
>>>> +        goto fail;
>>>> +    }
>>>>           s->cache_discards = true;
>>>>    @@ -896,13 +899,6 @@ 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 +908,7 @@ 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++) {
>>>> @@ -1055,13 +1050,8 @@ fail:
>>>>            }
>>>>               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]);
>>>> -        }
>>> But I realized something even more important as well: If you are
>>> modifying the current L1 table, you need to copy the result back to
>>> s->l1_table.
>>>
>> My fault.
>>> Sorry for having missed these in my previous review.
>>>
>>> Max
>>>
>>>>        }
>>>> -    if (l1_allocated)
>>>> -        g_free(l1_table);
>>>> +    g_free(l1_table);
>>>>        return ret;
>>>>    }
>