Re: [Qemu-devel] [PATCH v3 17/39] qcow2: Update l2_allocate() to support L2 slices

[Max Reitz]

On 2018-02-01 14:13, Alberto Garcia wrote:
> On Wed 31 Jan 2018 09:07:48 PM CET, Max Reitz wrote:
>> On 2018-01-26 15:59, Alberto Garcia wrote:
>>> This patch updates l2_allocate() to support the qcow2 cache returning
>>> L2 slices instead of full L2 tables.
>>>
>>> The old code simply gets an L2 table from the cache and initializes it
>>> with zeroes or with the contents of an existing table. With a cache
>>> that returns slices instead of tables the idea remains the same, but
>>> the code must now iterate over all the slices that are contained in an
>>> L2 table.
>>>
>>> Since now we're operating with slices the function can no longer
>>> return the newly-allocated table, so it's up to the caller to retrieve
>>> the appropriate L2 slice after calling l2_allocate() (note that with
>>> this patch the caller is still loading full L2 tables, but we'll deal
>>> with that in a separate patch).
>>>
>>> Signed-off-by: Alberto Garcia <address@hidden>
>>> ---
>>>  block/qcow2-cluster.c | 56 
>>> +++++++++++++++++++++++++++++++--------------------
>>>  1 file changed, 34 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>>> index 57349928a9..2a53c1dc5f 100644
>>> --- a/block/qcow2-cluster.c
>>> +++ b/block/qcow2-cluster.c
>>> @@ -264,11 +264,12 @@ int qcow2_write_l1_entry(BlockDriverState *bs, int 
>>> l1_index)
>>>   *
>>>   */
>>>  
>>> -static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t 
>>> **table)
>>> +static int l2_allocate(BlockDriverState *bs, int l1_index)
>>>  {
>>>      BDRVQcow2State *s = bs->opaque;
>>>      uint64_t old_l2_offset;
>>> -    uint64_t *l2_table = NULL;
>>> +    uint64_t *l2_slice = NULL;
>>> +    unsigned slice, slice_size2, n_slices;
>>
>> I'd personally prefer size_t, but oh well.
> 
> I don't see any reason not to use int / unsigned. The size of a slice is
> always <= cluster_size (an int), and both slice and n_slices are smaller
> than that.

Well, what's the reason to use unsigned? :-)

The type of the expression used to set slice_size2 simply is size_t.

>> However, I'm wondering whether this is the best approach.  The old L2
>> table is probably not going to be used after this function, so we're
>> basically polluting the cache here.  That was bad enough so far, but
>> now that actually means wasting multiple cache entries on it.
>>
>> Sure, the code is simpler this way.  But maybe it would still be
>> better to manually copy the data over from the old offset...  (As long
>> as it's not much more complicated.)
> 
> You mean bypassing the cache altogether?

Yes.

>     qcow2_cache_flush(bs, s->l2_table_cache);
>     new_table = g_malloc(s->cluster_size);
>     if (old_l2_offset & L1E_OFFSET_MASK) {
>         bdrv_pread(bs->file, old_l2_offset, new_table, s->cluster_size);
>     } else {
>         memset(new_table, 0, s->cluster_size);
>     }
>     bdrv_pwrite(bs->file, new_l2_offset, new_table, s->cluster_size);
>     g_free(new_table);
> 
> ??

Well, we wouldn't need to flush the whole table but only the parts of
the L2 table that we are about to copy.  And in theory we can omit even
that, because that old L2 table is not COPIED, so it can't be dirty anyway.

Max

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