Re: [Qemu-block] [RFC PATCH 0/8] qemu-img convert with copy offloading

[Paolo Bonzini]

On 05/04/2018 14:55, Stefan Hajnoczi wrote:
> bdrv_copy_file_range() will invoke bdrv_co_copy_file_range_src() on
> src[qcow2].  The qcow2 block driver will invoke
> bdrv_co_copy_file_range_src() on src[file].  The file-posix driver will
> invoke bdrv_co_copy_file_range_dst() on dst[raw].  The raw driver will
> invoke bdrv_co_copy_file_range_dst() on dst[file], which sees that
> src_bds (src[file]) is also file-posix and then goes ahead with
> copy_file_range(2).
> 
> In the case where src[qcow2] is on file-posix but dst[raw] is on iSCSI,
> the iSCSI .bdrv_co_copy_file_range_dst() call fails with -ENOTSUP and
> the block layer can fall back to a traditional copy operation.
> 
> With this approach src[qcow2] could take a lock or keep track of a
> serializing request struct so that other requests cannot interfere with
> the operation, and it's done in a natural way since we remain in the
> qcow2 function until the entire operation completes.  There's no need
> for bookkeeping structs or callbacks.

Could there be AB-BA deadlock if the guest attempts a concurrent copy
from A to B and from B to A?

Paolo

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