Re: [Qemu-devel] [PATCH v2 1/4] qcow2: multiple clusters write compressed

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Nov 16, 2017 at 07:54:55PM +0300, Anton Nefedov wrote:
> From: Pavel Butsykin <pbutsykin@virtuozzo.com>
> 
> At the moment, qcow2_co_pwritev_compressed can process the requests size
> less than or equal to one cluster. This patch added possibility to write
> compressed data in the QCOW2 more than one cluster. The implementation
> is simple, we just split large requests into separate clusters and write
> using existing functionality.
> 
> Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
> ---
>  block/qcow2.c | 73 ++++++++++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 52 insertions(+), 21 deletions(-)

Kevin: qcow2_alloc_compressed_cluster_offset() sets up an L2 table entry
before doing the compressed data write.  Can the following scenario
occur if there is another request racing with the compressed write?

1. Compressed cluster L2 table entry added to qcow2 in-memory cache
2. Another request forces cached L2 table to be written to disk
3. Power failure or crash before compressed data is written

Now there is an L2 table entry pointing to garbage data.  This violates
qcow2's data integrity model.

I'm not sure if compressed writes are safe...  It may have been okay for
qemu-img convert but the risk is increased when running a VM.

> diff --git a/block/qcow2.c b/block/qcow2.c
> index c276b24..f1e2759 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -3332,11 +3332,9 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
>      return 0;
>  }
>  
> -/* XXX: put compressed sectors first, then all the cluster aligned
> -   tables to avoid losing bytes in alignment */
>  static coroutine_fn int
> -qcow2_co_pwritev_compressed(BlockDriverState *bs, uint64_t offset,
> -                            uint64_t bytes, QEMUIOVector *qiov)
> +qcow2_co_pwritev_cluster_compressed(BlockDriverState *bs, uint64_t offset,
> +                                    uint64_t bytes, QEMUIOVector *qiov)
>  {
>      BDRVQcow2State *s = bs->opaque;
>      QEMUIOVector hd_qiov;
> @@ -3346,25 +3344,12 @@ qcow2_co_pwritev_compressed(BlockDriverState *bs, uint64_t offset,
>      uint8_t *buf, *out_buf;
>      int64_t cluster_offset;
>  
> -    if (bytes == 0) {
> -        /* align end of file to a sector boundary to ease reading with
> -           sector based I/Os */
> -        cluster_offset = bdrv_getlength(bs->file->bs);
> -        if (cluster_offset < 0) {
> -            return cluster_offset;
> -        }
> -        return bdrv_truncate(bs->file, cluster_offset, PREALLOC_MODE_OFF, NULL);
> -    }
> -
> -    if (offset_into_cluster(s, offset)) {
> -        return -EINVAL;
> -    }
> +    assert(bytes <= s->cluster_size);
> +    assert(!offset_into_cluster(s, offset));
>  
>      buf = qemu_blockalign(bs, s->cluster_size);
> -    if (bytes != s->cluster_size) {
> -        if (bytes > s->cluster_size ||
> -            offset + bytes != bs->total_sectors << BDRV_SECTOR_BITS)
> -        {
> +    if (bytes < s->cluster_size) {
> +        if (offset + bytes != bs->total_sectors << BDRV_SECTOR_BITS) {
>              qemu_vfree(buf);
>              return -EINVAL;
>          }
> @@ -3444,6 +3429,52 @@ fail:
>      return ret;
>  }
>  
> +/* XXX: put compressed sectors first, then all the cluster aligned
> +   tables to avoid losing bytes in alignment */
> +static coroutine_fn int
> +qcow2_co_pwritev_compressed(BlockDriverState *bs, uint64_t offset,
> +                            uint64_t bytes, QEMUIOVector *qiov)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    QEMUIOVector hd_qiov;
> +    uint64_t curr_off = 0;
> +    int ret;
> +
> +    if (bytes == 0) {
> +        /* align end of file to a sector boundary to ease reading with
> +           sector based I/Os */
> +        int64_t cluster_offset = bdrv_getlength(bs->file->bs);
> +        if (cluster_offset < 0) {
> +            return cluster_offset;
> +        }
> +        return bdrv_truncate(bs->file, cluster_offset, PREALLOC_MODE_OFF, NULL);
> +    }
> +
> +    if (offset_into_cluster(s, offset)) {
> +        return -EINVAL;
> +    }
> +
> +    qemu_iovec_init(&hd_qiov, qiov->niov);
> +    do {
> +        uint32_t chunk_size;
> +
> +        qemu_iovec_reset(&hd_qiov);
> +        chunk_size = MIN(bytes, s->cluster_size);
> +        qemu_iovec_concat(&hd_qiov, qiov, curr_off, chunk_size);
> +
> +        ret = qcow2_co_pwritev_cluster_compressed(bs, offset + curr_off,
> +                                                  chunk_size, &hd_qiov);
> +        if (ret < 0) {
> +            break;
> +        }
> +        curr_off += chunk_size;
> +        bytes -= chunk_size;
> +    } while (bytes);
> +    qemu_iovec_destroy(&hd_qiov);
> +
> +    return ret;
> +}
> +
>  static int make_completely_empty(BlockDriverState *bs)
>  {
>      BDRVQcow2State *s = bs->opaque;
> -- 
> 2.7.4
> 
> 

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