linux-kernel - Re: [PATCH] virtio_console: allocate inbufs in add

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Date:   Wed, 6 Nov 2019 16:41:33 +0100
From:   Laurent Vivier <lvivier@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Amit Shah <amit@...nel.org>,
        virtualization@...ts.linux-foundation.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] virtio_console: allocate inbufs in add_port() only if it
 is needed

On 06/11/2019 16:03, Michael S. Tsirkin wrote:
> On Wed, Nov 06, 2019 at 03:02:25PM +0100, Laurent Vivier wrote:
>> On 06/11/2019 14:56, Michael S. Tsirkin wrote:
>>> On Fri, Oct 18, 2019 at 06:47:18PM +0200, Laurent Vivier wrote:
>>>> When we hot unplug a virtserialport and then try to hot plug again,
>>>> it fails:
>>>>
>>>> (qemu) chardev-add socket,id=serial0,path=/tmp/serial0,server,nowait
>>>> (qemu) device_add virtserialport,bus=virtio-serial0.0,nr=2,\
>>>>                   chardev=serial0,id=serial0,name=serial0
>>>> (qemu) device_del serial0
>>>> (qemu) device_add virtserialport,bus=virtio-serial0.0,nr=2,\
>>>>                   chardev=serial0,id=serial0,name=serial0
>>>> kernel error:
>>>>   virtio-ports vport2p2: Error allocating inbufs
>>>> qemu error:
>>>>   virtio-serial-bus: Guest failure in adding port 2 for device \
>>>>                      virtio-serial0.0
>>>>
>>>> This happens because buffers for the in_vq are allocated when the port is
>>>> added but are not released when the port is unplugged.
>>>>
>>>> They are only released when virtconsole is removed (see a7a69ec0d8e4)
>>>>
>>>> To avoid the problem and to be symmetric, we could allocate all the buffers
>>>> in init_vqs() as they are released in remove_vqs(), but it sounds like
>>>> a waste of memory.
>>>>
>>>> Rather than that, this patch changes add_port() logic to only allocate the
>>>> buffers if the in_vq has available free slots.
>>>>
>>>> Fixes: a7a69ec0d8e4 ("virtio_console: free buffers after reset")
>>>> Cc: mst@...hat.com
>>>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
>>>> ---
>>>>  drivers/char/virtio_console.c | 17 +++++++++++------
>>>>  1 file changed, 11 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
>>>> index 7270e7b69262..77105166fe01 100644
>>>> --- a/drivers/char/virtio_console.c
>>>> +++ b/drivers/char/virtio_console.c
>>>> @@ -1421,12 +1421,17 @@ static int add_port(struct ports_device *portdev, u32 id)
>>>>  	spin_lock_init(&port->outvq_lock);
>>>>  	init_waitqueue_head(&port->waitqueue);
>>>>  
>>>> -	/* Fill the in_vq with buffers so the host can send us data. */
>>>> -	nr_added_bufs = fill_queue(port->in_vq, &port->inbuf_lock);
>>>> -	if (!nr_added_bufs) {
>>>> -		dev_err(port->dev, "Error allocating inbufs\n");
>>>> -		err = -ENOMEM;
>>>> -		goto free_device;
>>>> +	/* if the in_vq has not already been filled (the port has already been
>>>> +	 * used and unplugged), fill the in_vq with buffers so the host can
>>>> +	 * send us data.
>>>> +	 */
>>>> +	if (port->in_vq->num_free != 0) {
>>>> +		nr_added_bufs = fill_queue(port->in_vq, &port->inbuf_lock);
>>>> +		if (!nr_added_bufs) {
>>>> +			dev_err(port->dev, "Error allocating inbufs\n");
>>>> +			err = -ENOMEM;
>>>> +			goto free_device;
>>>> +		}
>>>>  	}
>>>>  
>>>>  	if (is_rproc_serial(port->portdev->vdev))
>>>
>>> Well fill_queue will just add slots as long as it can.
>>> So on a full queue it does nothing. How does this patch help?
>>
>> Yes, but in this case it returns 0 and so add_port() fails and exits
>> with -ENOMEM and the device is freed. It's what this patch tries to avoid.
>>
>> Thanks,
>> Laurent
> 
> Oh I see. However it's a bit asymmetrical to special case ring full.
> How about making fill_queue return int and testing return code for
> -ENOSPC instead? Will also help propagate errors correctly.

Good idea. I'm going to propose a new patch.

> And I guess CC stable?
Sure.

Thanks,
Laurent