Re: [PATCH v3 1/4] block/io: accept NULL qiov in bdrv_pad_request

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Mar 22, 2024 at 10:50:06AM +0100, Fiona Ebner wrote:
> From: Stefan Reiter <s.reiter@proxmox.com>
> 
> Some operations, e.g. block-stream, perform reads while discarding the
> results (only copy-on-read matters). In this case, they will pass NULL
> as the target QEMUIOVector, which will however trip bdrv_pad_request,
> since it wants to extend its passed vector. In particular, this is the
> case for the blk_co_preadv() call in stream_populate().
> 
> If there is no qiov, no operation can be done with it, but the bytes
> and offset still need to be updated, so the subsequent aligned read
> will actually be aligned and not run into an assertion failure.
> 
> In particular, this can happen when the request alignment of the top
> node is larger than the allocated part of the bottom node, in which
> case padding becomes necessary. For example:
> 
> > ./qemu-img create /tmp/backing.qcow2 -f qcow2 64M -o cluster_size=32768
> > ./qemu-io -c "write -P42 0x0 0x1" /tmp/backing.qcow2
> > ./qemu-img create /tmp/top.qcow2 -f qcow2 64M -b /tmp/backing.qcow2 -F qcow2
> > ./qemu-system-x86_64 --qmp stdio \
> > --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/top.qcow2 \
> > <<EOF
> > {"execute": "qmp_capabilities"}
> > {"execute": "blockdev-add", "arguments": { "driver": "compress", "file": "node0", "node-name": "node1" } }
> > {"execute": "block-stream", "arguments": { "job-id": "stream0", "device": "node1" } }
> > EOF
> 
> Originally-by: Stefan Reiter <s.reiter@proxmox.com>
> Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
> [FE: do update bytes and offset in any case
>      add reproducer to commit message]
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
> 
> No changes in v3.
> No changes in v2.
> 
>  block/io.c | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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