Re: [Qemu-devel] [PATCHv2] block: add native support for NFS

[Peter Lieven <pl@kamp.de>]

On 20.12.2013 16:54, Stefan Hajnoczi wrote:
> On Fri, Dec 20, 2013 at 4:49 PM, Peter Lieven <pl@kamp.de> wrote:
>> On 20.12.2013 16:30, Stefan Hajnoczi wrote:
>>> On Fri, Dec 20, 2013 at 3:43 PM, Peter Lieven <pl@kamp.de> wrote:
>>>> On 20.12.2013 15:38, Stefan Hajnoczi wrote:
>>>>> On Fri, Dec 20, 2013 at 3:07 PM, Peter Lieven <pl@kamp.de> wrote:
>>>>>> On 20.12.2013 14:57, Stefan Hajnoczi wrote:
>>>>>>> On Fri, Dec 20, 2013 at 1:53 PM, Peter Lieven <pl@kamp.de> wrote:
>>>>>>>> On 20.12.2013 13:19, Stefan Hajnoczi wrote:
>>>>>>>>> On Fri, Dec 20, 2013 at 10:48:41AM +0100, Peter Lieven wrote:
>>>>>>>>>> On 17.12.2013 17:47, Stefan Hajnoczi wrote:
>>>>>>>>>>> On Tue, Dec 17, 2013 at 10:15:25AM +0100, Peter Lieven wrote:
>>>>>>>>>>>> +    /* set to -ENOTSUP since bdrv_allocated_file_size is only
>>>>>>>>>>>> used
>>>>>>>>>>>> +     * in qemu-img open. So we can use the cached value for
>>>>>>>>>>>> allocate
>>>>>>>>>>>> +     * filesize obtained from fstat at open time */
>>>>>>>>>>>> +    client->allocated_file_size = -ENOTSUP;
>>>>>>>>>>> Can you implement this fully?  By stubbing it out like this we
>>>>>>>>>>> won't
>>>>>>>>>>> be
>>>>>>>>>>> able to call get_allocated_file_size() at runtime in the future
>>>>>>>>>>> without
>>>>>>>>>>> updating the nfs block driver code.  It's just an fstat call,
>>>>>>>>>>> shouldn't
>>>>>>>>>>> be too hard to implement properly :).
>>>>>>>>>> It seems I have to leave it as is currently.
>>>>>>>>>> bdrv_get_allocated_file_size
>>>>>>>>>> is not in a coroutine context. I get coroutine yields to no one.
>>>>>>>>> Create a coroutine and pump the event loop until it has reached
>>>>>>>>> completion:
>>>>>>>>>
>>>>>>>>> co = qemu_coroutine_create(my_coroutine_fn, ...);
>>>>>>>>> qemu_coroutine_enter(co, foo);
>>>>>>>>> while (!complete) {
>>>>>>>>>          qemu_aio_wait();
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> See block.c for similar examples.
>>>>>>>> Wouldn't it make sense to make this modification to
>>>>>>>> bdrv_get_allocated_file_size in
>>>>>>>> block.c rather than in client/nfs.c and in the future potentially
>>>>>>>> other
>>>>>>>> drivers?
>>>>>>>>
>>>>>>>> If yes, I would ask you to take v3 of the NFS protocol patch and I
>>>>>>>> promise
>>>>>>>> to send
>>>>>>>> a follow up early next year to make this modification to block.c and
>>>>>>>> change
>>>>>>>> block/nfs.c
>>>>>>>> and other implementations to be a coroutine_fn.
>>>>>>> .bdrv_get_allocated_file_size() implementations in other block drivers
>>>>>>> are synchronous.  Making the block driver interface use coroutines
>>>>>>> would be wrong unless all the block drivers were updated to use
>>>>>>> coroutines too.
>>>>>> I can do that. I think its not too complicated because all those
>>>>>> implementations do not rely on callbacks. It should be possible
>>>>>> to just rename the existing implemenations to lets say
>>>>>> .bdrv_co_get_allocated_file_size and call them inside a coroutine.
>>>>> No, that would be wrong because coroutine functions should not block.
>>>>> The point of coroutines is that if they cannot proceed they must yield
>>>>> so the event loop regains control.  If you simply rename the function
>>>>> to _co_ then they will block the event loop and not be true coroutine
>>>>> functions.
>>>>>
>>>>>>> Can you just call nfs_fstat() (the sync libnfs interface)?
>>>>>> I can only do that if its guaranteed that no other requests are in
>>>>>> flight
>>>>>> otherwise it will mess up.
>>>>> How will it mess up?
>>>> The sync calls into libnfs are just wrappers around the async calls.
>>>> The problem is that this wrapper will handle all the callbacks for the
>>>> in-flight requests and they will never return.
>>> So back to my original suggestion to use a qemu_aio_wait() loop in
>>> block/nfs.c?
>> sorry, I cannot follow you. but maybe this here is a short solution.
>> question
>> is, what will happen when there are pending requests which invoke callbacks.
>> will we end up returning from qemu_aio_wait? in the qemu-img info case
>> this here works:
>>
>> static int64_t nfs_get_allocated_file_size(BlockDriverState *bs)
>> {
>>
>>      NFSClient *client = bs->opaque;
>>      NFSRPC task = {0};
>>      struct stat st;
>>
>>      task.st = &st;
>>      if (nfs_fstat_async(client->context, client->fh, nfs_co_generic_cb,
>>                          &task) != 0) {
>>          return -ENOMEM;
>>      }
>>
>>      while (!task.complete) {
>>          nfs_set_events(client);
>>          qemu_aio_wait();
>>      }
>>
>>      return (task.status < 0 ? task.status : st.st_blocks * st.st_blksize);
>> }
> Yes, that's exactly what I had in mind.
>
> Yes, other callbacks will get called and requests will complete in
> this event loop.  That can be a problem in some scenarios but should
> be okay with bdrv_get_allocated_file_size().
ok I will send v4 and then prepare for the holidays ;-)

Peter