Re: [Qemu-devel] [PATCH v11 09/10] qcow2: skip writing zero buffers to empty COW areas

[Vladimir Sementsov-Ogievskiy]

24.12.2018 11:21, Anton Nefedov wrote:
> On 21/12/2018 7:16 PM, Vladimir Sementsov-Ogievskiy wrote:
>> 18.12.2018 10:57, Anton Nefedov wrote:
>>> If COW areas of the newly allocated clusters are zeroes on the backing 
>>> image,
>>> efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the 
>>> whole
>>> cluster instead of writing explicit zero buffers later in perform_cow().
>>>
>>> iotest 060:
>>> write to the discarded cluster does not trigger COW anymore.
>>> Use a backing image instead.
>>>
>>> Signed-off-by: Anton Nefedov <address@hidden>
>>> ---
>>>     qapi/block-core.json       |  4 +-
>>>     block/qcow2.h              |  6 +++
>>>     block/qcow2-cluster.c      |  2 +-
>>>     block/qcow2.c              | 89 +++++++++++++++++++++++++++++++++++++-
>>>     block/trace-events         |  1 +
>>>     tests/qemu-iotests/060     |  7 ++-
>>>     tests/qemu-iotests/060.out |  5 ++-
>>>     7 files changed, 108 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>>> index 762000f31f..204528b3f6 100644
>>> --- a/qapi/block-core.json
>>> +++ b/qapi/block-core.json
>>> @@ -3009,6 +3009,8 @@
>>>     #
>>>     # @cor_write: a write due to copy-on-read (since 2.11)
>>>     #
>>> +# @cluster_alloc_space: an allocation of file space for a cluster (since 
>>> 4.0)
>>> +#
>>>     # Since: 2.9
>>>     ##
>>>     { 'enum': 'BlkdebugEvent', 'prefix': 'BLKDBG',
>>> @@ -3027,7 +3029,7 @@
>>>                 'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
>>>                 'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
>>>                 'l1_shrink_write_table', 'l1_shrink_free_l2_clusters',
>>> -            'cor_write'] }
>>> +            'cor_write', 'cluster_alloc_space'] }
>>>     
>>>     ##
>>>     # @BlkdebugInjectErrorOptions:
>>> diff --git a/block/qcow2.h b/block/qcow2.h
>>> index a98d24500b..32d2c04bfa 100644
>>> --- a/block/qcow2.h
>>> +++ b/block/qcow2.h
>>> @@ -386,6 +386,12 @@ typedef struct QCowL2Meta
>>>          */
>>>         Qcow2COWRegion cow_end;
>>>     
>>> +    /*
>>> +     * Indicates that COW regions are already handled and do not require
>>> +     * any more processing.
>>> +     */
>>> +    bool skip_cow;
>>> +
>>>         /**
>>
>> hmm, around it, all comments starts from '/**', so, I think, yours should 
>> too.
>> (this note doesn't touch your other comments)
>>
> 
> that triggers a warning in checkpatch.pl
> 
>     WARNING: Block comments use a leading /* on a separate line

I thing, it is the case, when we should ignore this warning.

> 
>>>          * The I/O vector with the data from the actual guest write request.
>>>          * If non-NULL, this is meant to be merged together with the data
>>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>>> index e2737429f5..23e0702027 100644
>>> --- a/block/qcow2-cluster.c
>>> +++ b/block/qcow2-cluster.c
>>> @@ -806,7 +806,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta 
>>> *m)
>>>         assert(start->offset + start->nb_bytes <= end->offset);
>>>         assert(!m->data_qiov || m->data_qiov->size == data_bytes);
>>>     
>>> -    if (start->nb_bytes == 0 && end->nb_bytes == 0) {
>>> +    if ((start->nb_bytes == 0 && end->nb_bytes == 0) || m->skip_cow) {
>>>             return 0;
>>>         }
>>>     
>>> diff --git a/block/qcow2.c b/block/qcow2.c
>>> index 4897abae5e..161b935962 100644
>>> --- a/block/qcow2.c
>>> +++ b/block/qcow2.c
>>> @@ -2021,6 +2021,11 @@ static bool merge_cow(uint64_t offset, unsigned 
>>> bytes,
>>>                 continue;
>>>             }
>>>     
>>> +        /* If COW regions are handled already, skip this too */
>>> +        if (m->skip_cow) {
>>> +            continue;
>>> +        }
>>> +
>>>             /* The data (middle) region must be immediately after the
>>>              * start region */
>>>             if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
>>> @@ -2046,6 +2051,77 @@ static bool merge_cow(uint64_t offset, unsigned 
>>> bytes,
>>>         return false;
>>>     }
>>>     
>>> +static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t 
>>> bytes)
>>> +{
>>> +    int64_t nr;
>>> +    return !bytes ||
>>> +        (!bdrv_is_allocated_above(bs, NULL, offset, bytes, &nr) && nr == 
>>> bytes);
>>> +}
>>> +
>>> +static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
>>> +{
>>> +    /*
>>> +     * This check is designed for optimization shortcut so it must be
>>> +     * efficient.
>>> +     * Instead of is_zero(), use is_unallocated() as it is faster (but not
>>> +     * as accurate and can result in false negatives).
>>> +     */
>>> +    return is_unallocated(bs, m->offset + m->cow_start.offset,
>>
>> here you add m->cow_start.offset ...[1]
>>
>>> +                          m->cow_start.nb_bytes) &&
>>> +           is_unallocated(bs, m->offset + m->cow_end.offset,
>>> +                          m->cow_end.nb_bytes);
>>> +}
>>> +
>>> +static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
>>> +{
>>> +    BDRVQcow2State *s = bs->opaque;
>>> +    QCowL2Meta *m;
>>> +
>>> +    if (!(bs->file->bs->supported_zero_flags & BDRV_REQ_ALLOCATE)) {
>>> +        return 0;
>>> +    }
>>> +
>>> +    if (bs->encrypted) {
>>> +        return 0;
>>> +    }
>>> +
>>> +    for (m = l2meta; m != NULL; m = m->next) {
>>> +        int ret;
>>> +
>>> +        if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
>>> +            continue;
>>> +        }
>>> +
>>> +        if (!is_zero_cow(bs, m)) {
>>> +            continue;
>>> +        }
>>> +
>>> +        /*
>>> +         * Conventional place for qcow2_pre_write_overlap_check() but in 
>>> this
>>> +         * case it is already done for these clusters
>>> +         */
>>
>> to be honest, check is done for original request not, not for cow_regions, 
>> and,
>> on the other hand,
>> do_perform_cow_write _does_ the check.
>>
>> So, I see two consistent ways:
>> 1. you just add check here, like in do_perform_cow_write
>> 2. improve check in qcow2_co_pwritev to cover the whole area, and drop 
>> checks from
>>       perform_cow.
>>
>> I prefer 1, as it more helpful in case of bugs in the code (just check 
>> exactly what
>> you are going to write in the next write).
>>
> 
> ok
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 161b935962..05a7cbebbd 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2096,16 +2096,18 @@ static int handle_alloc_space(BlockDriverState
> *bs, QCowL2Meta *l2meta)
>                continue;
>            }
> 
> -        /*
> -         * Conventional place for qcow2_pre_write_overlap_check() but
> in this
> -         * case it is already done for these clusters
> -         */
> -
> -        BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
>            /*
>             * instead of writing zero COW buffers,
>             * efficiently zero out the whole clusters
>             */
> +
> +        ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset,
> +                                            m->nb_clusters *
> s->cluster_size);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +
> +        BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);

ok

>            ret = bdrv_co_pwrite_zeroes(bs->file, m->alloc_offset,
>                                        m->nb_clusters * s->cluster_size,
>                                        BDRV_REQ_ALLOCATE);
> 
>>> +
>>> +        BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
>>> +        /*
>>> +         * instead of writing zero COW buffers,
>>> +         * efficiently zero out the whole clusters
>>> +         */
>>> +        ret = bdrv_co_pwrite_zeroes(bs->file, m->alloc_offset,
>>
>> [1]... and here - not.
>> And perform_cow uses start too.
>> hmm..
>>
>> On the other hand, here we just want to zero all allocated clusters, it is 
>> not related
>> to cow regions. So, ok.
>>
>>> +                                    m->nb_clusters * s->cluster_size,
>>> +                                    BDRV_REQ_ALLOCATE);
>>> +        if (ret < 0) {
>>> +            if (ret != -ENOTSUP && ret != -EAGAIN) {
>>> +                return ret;
>>> +            }
>>> +            continue;
>>> +        }
>>> +
>>> +        trace_qcow2_skip_cow(qemu_coroutine_self(), m->offset, 
>>> m->nb_clusters);
>>> +        m->skip_cow = true;
>>> +    }
>>> +    return 0;
>>> +}
>>> +
>>
>> [...]
>>
>>
>> with pre-write-check added:
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>
>>
>>


-- 
Best regards,
Vladimir