Re: [Qemu-devel] [PATCH 03/11] qcow2: add qcow2_co_write_compressed

[Pavel Butsykin <pbutsykin@virtuozzo.com>]

On 28.06.2016 14:30, Kevin Wolf wrote:
> Am 31.05.2016 um 11:15 hat Denis V. Lunev geschrieben:
>> From: Pavel Butsykin <pbutsykin@virtuozzo.com>
>>
>> Added implementation of the qcow2_co_write_compressed function that
>> will allow us to safely use compressed writes for the qcow2 from running VMs.
>>
>> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Jeff Cody <jcody@redhat.com>
>> CC: Markus Armbruster <armbru@redhat.com>
>> CC: Eric Blake <eblake@redhat.com>
>> CC: John Snow <jsnow@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> ---
>>   block/qcow2.c | 89 ++++++++++++++++++++++++++++++++++-------------------------
>>   1 file changed, 52 insertions(+), 37 deletions(-)
>>
>> diff --git a/block/qcow2.c b/block/qcow2.c
>> index c9306a7..38caa66 100644
>> --- a/block/qcow2.c
>> +++ b/block/qcow2.c
>> @@ -2535,13 +2535,16 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
>>
>>   /* XXX: put compressed sectors first, then all the cluster aligned
>>      tables to avoid losing bytes in alignment */
>> -static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
>> -                                  const uint8_t *buf, int nb_sectors)
>> +static coroutine_fn int
>> +qcow2_co_write_compressed(BlockDriverState *bs, int64_t sector_num,
>> +                          int nb_sectors, QEMUIOVector *qiov)
>>   {
>>       BDRVQcow2State *s = bs->opaque;
>> +    QEMUIOVector hd_qiov;
>> +    struct iovec iov;
>>       z_stream strm;
>>       int ret, out_len;
>> -    uint8_t *out_buf;
>> +    uint8_t *buf, *out_buf;
>>       uint64_t cluster_offset;
>>
>>       if (nb_sectors == 0) {
>> @@ -2551,29 +2554,25 @@ static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
>>           return bdrv_truncate(bs->file->bs, cluster_offset);
>>       }
>>
>> +    buf = qemu_blockalign(bs, s->cluster_size);
>>       if (nb_sectors != s->cluster_sectors) {
>> -        ret = -EINVAL;
>> -
>> -        /* Zero-pad last write if image size is not cluster aligned */
>> -        if (sector_num + nb_sectors == bs->total_sectors &&
>> -            nb_sectors < s->cluster_sectors) {
>> -            uint8_t *pad_buf = qemu_blockalign(bs, s->cluster_size);
>> -            memset(pad_buf, 0, s->cluster_size);
>> -            memcpy(pad_buf, buf, nb_sectors * BDRV_SECTOR_SIZE);
>> -            ret = qcow2_write_compressed(bs, sector_num,
>> -                                         pad_buf, s->cluster_sectors);
>> -            qemu_vfree(pad_buf);
>> +        if (nb_sectors > s->cluster_sectors ||
>> +            sector_num + nb_sectors != bs->total_sectors)
>> +        {
>> +            qemu_vfree(buf);
>> +            return -EINVAL;
>>           }
>> -        return ret;
>> +        /* Zero-pad last write if image size is not cluster aligned */
>> +        memset(buf, 0, s->cluster_size);
>>       }
>> +    qemu_iovec_to_buf(qiov, 0, buf, qiov->size);
>
> This looks less related to the new interface, but more like an unrelated
> (but still worthwhile) cleanup to avoid the recursion.
>
> Can we separate this out as a cleanup patch before this one?
>
We can :)

> Also, the last parameter of qemu_iovec_to_buf() should be
> s->cluster_size, it's the buffer size and not the qiov size.
> Additionally, we may want to assert(qiov->size == s->cluster_size).

It is not necessary, the qiov size can be less than s->cluster_size. In
this case, the remaining part of the cluster is filled with zeros.

>
>>       out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
>>
>>       /* best compression, small window, no zlib header */
>>       memset(&strm, 0, sizeof(strm));
>> -    ret = deflateInit2(&strm, Z_DEFAULT_COMPRESSION,
>> -                       Z_DEFLATED, -12,
>> -                       9, Z_DEFAULT_STRATEGY);
>> +    ret = deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED,
>> +                       -12, 9, Z_DEFAULT_STRATEGY);
>
> Unrelated reformatting? Let's drop this, so the semantic changes in the
> patch become more visible.
>
ok

>>       if (ret != 0) {
>>           ret = -EINVAL;
>>           goto fail;
>> @@ -2595,34 +2594,50 @@ static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
>>       deflateEnd(&strm);
>>
>>       if (ret != Z_STREAM_END || out_len >= s->cluster_size) {
>> +        iov = (struct iovec) {
>> +            .iov_base   = buf,
>> +            .iov_len    = out_len,
>> +        };
>> +        qemu_iovec_init_external(&hd_qiov, &iov, 1);
>>           /* could not compress: write normal cluster */
>> -        ret = bdrv_write(bs, sector_num, buf, s->cluster_sectors);
>> +        ret = qcow2_co_writev(bs, sector_num, s->cluster_sectors, &hd_qiov);
>
> Now that it's qcow2_co_pwritev(), you can probably just use the existing
> qiov.
>
>>           if (ret < 0) {
>>               goto fail;
>>           }
>> -    } else {
>> -        cluster_offset = qcow2_alloc_compressed_cluster_offset(bs,
>> -            sector_num << 9, out_len);
>> -        if (!cluster_offset) {
>> -            ret = -EIO;
>> -            goto fail;
>> -        }
>> -        cluster_offset &= s->cluster_offset_mask;
>> +        goto success;
>> +    }
>>
>> -        ret = qcow2_pre_write_overlap_check(bs, 0, cluster_offset, out_len);
>> -        if (ret < 0) {
>> -            goto fail;
>> -        }
>> +    qemu_co_mutex_lock(&s->lock);
>> +    cluster_offset = \
>
> That backslash isn't necessary.
>
I know it's just a marker.

>> +        qcow2_alloc_compressed_cluster_offset(bs, sector_num << 9, out_len);
>> +    if (!cluster_offset) {
>> +        qemu_co_mutex_unlock(&s->lock);
>> +        ret = -EIO;
>> +        goto fail;
>> +    }
>> +    cluster_offset &= s->cluster_offset_mask;
>
> Kevin
>