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

[Stefan Hajnoczi]

On Fri, Dec 20, 2013 at 4:49 PM, Peter Lieven <address@hidden> wrote:
> On 20.12.2013 16:30, Stefan Hajnoczi wrote:
>>
>> On Fri, Dec 20, 2013 at 3:43 PM, Peter Lieven <address@hidden> wrote:
>>>
>>> On 20.12.2013 15:38, Stefan Hajnoczi wrote:
>>>>
>>>> On Fri, Dec 20, 2013 at 3:07 PM, Peter Lieven <address@hidden> wrote:
>>>>>
>>>>> On 20.12.2013 14:57, Stefan Hajnoczi wrote:
>>>>>>
>>>>>> On Fri, Dec 20, 2013 at 1:53 PM, Peter Lieven <address@hidden> 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().

Stefan