linux-kernel - Re: [PATCH v2] virtio_console: allocate inbufs in add

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Date:   Wed, 13 Nov 2019 14:28:42 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Laurent Vivier <lvivier@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Amit Shah <amit@...nel.org>,
        virtualization@...ts.linux-foundation.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] virtio_console: allocate inbufs in add_port() only if
 it is needed

On Wed, Nov 13, 2019 at 05:37:34PM +0100, Laurent Vivier wrote:
> On 13/11/2019 16:22, Michael S. Tsirkin wrote:
> > On Wed, Nov 13, 2019 at 10:21:11AM -0500, Michael S. Tsirkin wrote:
> >> On Wed, Nov 13, 2019 at 04:00:56PM +0100, 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 ignore ENOSPC
> >>> error in fill_queue(), which means queue has already been filled.
> >>>
> >>> Fixes: a7a69ec0d8e4 ("virtio_console: free buffers after reset")
> >>> Cc: mst@...hat.com
> >>> Cc: stable@...r.kernel.org
> >>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
> >>> ---
> >>>
> >>> Notes:
> >>>     v2: making fill_queue return int and testing return code for -ENOSPC
> >>>
> >>>  drivers/char/virtio_console.c | 24 +++++++++---------------
> >>>  1 file changed, 9 insertions(+), 15 deletions(-)
> >>>
> >>> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> >>> index 7270e7b69262..9e6534fd1aa4 100644
> >>> --- a/drivers/char/virtio_console.c
> >>> +++ b/drivers/char/virtio_console.c
> >>> @@ -1325,24 +1325,24 @@ static void set_console_size(struct port *port, u16 rows, u16 cols)
> >>>  	port->cons.ws.ws_col = cols;
> >>>  }
> >>>  
> >>> -static unsigned int fill_queue(struct virtqueue *vq, spinlock_t *lock)
> >>> +static int fill_queue(struct virtqueue *vq, spinlock_t *lock)
> >>>  {
> >>>  	struct port_buffer *buf;
> >>> -	unsigned int nr_added_bufs;
> >>> +	int nr_added_bufs;
> >>>  	int ret;
> >>>  
> >>>  	nr_added_bufs = 0;
> >>>  	do {
> >>>  		buf = alloc_buf(vq->vdev, PAGE_SIZE, 0);
> >>>  		if (!buf)
> >>> -			break;
> >>> +			return -ENOMEM;
> >>>  
> >>>  		spin_lock_irq(lock);
> >>>  		ret = add_inbuf(vq, buf);
> >>>  		if (ret < 0) {
> >>>  			spin_unlock_irq(lock);
> >>>  			free_buf(buf, true);
> >>> -			break;
> >>> +			return ret;
> >>>  		}
> >>>  		nr_added_bufs++;
> >>>  		spin_unlock_irq(lock);
> > 
> > So actually, how about handling ENOSPC specially here, and
> > returning success? After all queue is full as requested ...
> 
> I think it's interesting to return -ENOSPC to manage it as a real error
> in virtcons_probe() as in this function the queue should not be already
> full (is this right?) and to return the real error code.
> 
> Thanks,
> Laurent

OK then. Pls add comments.