Re: [PATCH v11 02/13] copy-on-read: add filter append/drop functions

[Max Reitz]

On 14.10.20 16:28, Andrey Shinkevich wrote:
> On 14.10.2020 13:44, Max Reitz wrote:
>> On 12.10.20 19:43, Andrey Shinkevich wrote:
>>> Provide API for the COR-filter insertion/removal.
>>> Also, drop the filter child permissions for an inactive state when the
>>> filter node is being removed.
>>>
>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   block/copy-on-read.c | 88
>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>   block/copy-on-read.h | 35 +++++++++++++++++++++
>>>   2 files changed, 123 insertions(+)
>>>   create mode 100644 block/copy-on-read.h
>>>
>>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>>> index cb03e0f..bcccf0f 100644
>>> --- a/block/copy-on-read.c
>>> +++ b/block/copy-on-read.c
>>
>> [...]
>>
>>> @@ -159,4 +188,63 @@ static void bdrv_copy_on_read_init(void)
>>>       bdrv_register(&bdrv_copy_on_read);
>>>   }
>>>   +
>>> +BlockDriverState *bdrv_cor_filter_append(BlockDriverState *bs,
>>> +                                         QDict *node_options,
>>> +                                         int flags, Error **errp)
>>
>> I had hoped you could make this a generic block layer function. :(
>>
>> (Because it really is rather generic)
>>
>> *shrug*
> 
> Actually, I did (and still can do) that for the 'append node' function
> only but not for the 'drop node' one so far...

Ah, yeah.

> diff --git a/block.c b/block.c
> index 11ab55f..f41e876 100644
> --- a/block.c
> +++ b/block.c
> @@ -4669,6 +4669,55 @@ static void bdrv_delete(BlockDriverState *bs)
>      g_free(bs);
>  }
> 
> +BlockDriverState *bdrv_insert_node(BlockDriverState *bs, QDict
> *node_options,
> +                                   int flags, Error **errp)
> +{
> +    BlockDriverState *new_node_bs;
> +    Error *local_err = NULL;
> +
> +    new_node_bs =  bdrv_open(NULL, NULL, node_options, flags, errp);
> +    if (new_node_bs == NULL) {
> +        error_prepend(errp, "Could not create node: ");
> +        return NULL;
> +    }
> +
> +    bdrv_drained_begin(bs);
> +    bdrv_replace_node(bs, new_node_bs, &local_err);
> +    bdrv_drained_end(bs);
> +
> +    if (local_err) {
> +        bdrv_unref(new_node_bs);
> +        error_propagate(errp, local_err);
> +        return NULL;
> +    }
> +
> +    return new_node_bs;
> +}
> +
> +void bdrv_remove_node(BlockDriverState *bs)
> +{
> +    BdrvChild *child;
> +    BlockDriverState *inferior_bs;
> +
> +    child = bdrv_filter_or_cow_child(bs);
> +    if (!child) {
> +        return;
> +    }
> +    inferior_bs = child->bs;
> +
> +    /* Retain the BDS until we complete the graph change. */
> +    bdrv_ref(inferior_bs);
> +    /* Hold a guest back from writing while permissions are being
> reset. */
> +    bdrv_drained_begin(inferior_bs);
> +    /* Refresh permissions before the graph change. */
> +    bdrv_child_refresh_perms(bs, child, &error_abort);
> +    bdrv_replace_node(bs, inferior_bs, &error_abort);
> +
> +    bdrv_drained_end(inferior_bs);
> +    bdrv_unref(inferior_bs);
> +    bdrv_unref(bs);
> +}
> 
> So, it is an intermediate solution in this patch of the series. I am
> going to make both functions generic once Vladimir overhauls the QEMU
> permission update system. Otherwise, the COR-filter node cannot be
> removed from the backing chain gracefully.

True.

> Thank you for your r-b. If the next version comes, I can move the
> 'append node' function only to the generic layer.

Thanks!  Even if you decide against it, I’m happy as long as there are
plans to perhaps make it generic at some point.

(I’m just afraid this function might be useful in some other case, too,
and we forget that it exists here already, and then end up
reimplementing it.)

Max

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