Re: [PATCH] block/backup-top: fix flags handling

[Eric Blake <eblake@redhat.com>]

On 1/31/20 12:48 PM, Vladimir Sementsov-Ogievskiy wrote:
> backup-top "supports" write-unchanged, by skipping CBW operation in
> backup_top_co_pwritev. But it forgets to do the same in
> backup_top_co_pwrite_zeroes, as well as declare support for
> BDRV_REQ_WRITE_UNCHANGED.
> 
> Fix this, and, while being here, declare also support for flags
> supported by source child.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/backup-top.c | 28 ++++++++++++++++++----------
>   1 file changed, 18 insertions(+), 10 deletions(-)


> @@ -197,6 +201,10 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
>   
>       top->total_sectors = source->total_sectors;
>       state = top->opaque;
> +    top->supported_write_flags =
> +        BDRV_REQ_WRITE_UNCHANGED | source->supported_write_flags;
> +    top->supported_zero_flags =
> +        BDRV_REQ_WRITE_UNCHANGED | source->supported_zero_flags;

Elsewhere, in block/filter-compress.c we do:

     bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
         (BDRV_REQ_FUA & bs->file->bs->supported_write_flags);

     bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
         ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
             bs->file->bs->supported_zero_flags);

That's slightly more robust (if the block layer adds new BDRV_REQ_ bits, 
we don't have to revisit filter-compress.c to decide if blindly exposing 
those bits breaks for some reason, but rahter DO have to amend the line 
to opt-in to supporting the new bits).  Whereas your code does NOT need 
editing if passing on the new bit is safe, but risks a subtle breakage 
if we forget to filter out the new bit when passing it on would be 
unsafe.  I tend to lean towards safety and opt-in over blind 
pass-through that works with the current set of defined bits, but not 
enough to withhold:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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