Re: [PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Kevin Wolf <kwolf@redhat.com>]

Am 22.04.2020 um 17:33 hat Eric Blake geschrieben:
> On 4/22/20 10:21 AM, Kevin Wolf wrote:
> > If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
> > qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
> > undo any previous preallocation, but just adds the zero flag to all
> > relevant L2 entries. If an external data file is in use, a write_zeroes
> > request to the data file is made instead.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   block/qcow2.c | 30 ++++++++++++++++++++++++++++++
> >   1 file changed, 30 insertions(+)
> > 
> 
> > @@ -4214,6 +4215,35 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
> >           g_assert_not_reached();
> >       }
> > +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
> > +        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
> > +        uint64_t zero_end = QEMU_ALIGN_UP(offset, s->cluster_size);
> 
> This rounds up beyond the new size...
> 
> > +
> > +        /* Use zero clusters as much as we can */
> > +        ret = qcow2_cluster_zeroize(bs, zero_start, zero_end - zero_start, 0);
> 
> and then requests that the extra be zeroed.  Does that always work, even
> when it results in pdrv_co_pwrite_zeroes beyond the end of s->data_file?

You mean the data_file_is_raw() path in qcow2_cluster_zeroize()? It's
currently not a code path that is run because we only set
BDRV_REQ_ZERO_WRITE for truncate if the image has a backing file, and
data_file_is_raw() doesn't work with backing files.

But hypothetically, if someone called truncate with BDRV_REQ_ZERO_WRITE
for such a file, I think it would fail.

> If so,
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>
> 
> otherwise, you may have to treat the tail specially, the same way you
> treated an unaligned head.

Actually, do I even need to round the tail?

    /* Caller must pass aligned values, except at image end */
    assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
    assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
           end_offset == bs->total_sectors << BDRV_SECTOR_BITS);

So qcow2_cluster_zeroize() seems to accept the unaligned tail. It would
still set the zero flag for the partial last cluster and for the
external data file, bdrv_co_pwrite_zeroes() would have the correct size.

Kevin