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Subject: Re: [PATCH] virtio_console: allocate inbufs in add_port() only if it
 is needed
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Amit Shah <amit@kernel.org>,
        virtualization@lists.linux-foundation.org,
        Arnd Bergmann <arnd@arndb.de>
References: <20191018164718.15999-1-lvivier@redhat.com>
 <20191106085548-mutt-send-email-mst@kernel.org>
From:   Laurent Vivier <lvivier@redhat.com>
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Message-ID: <83d88904-1626-8dd6-9e5c-7abcee27bcd0@redhat.com>
Date:   Wed, 6 Nov 2019 15:02:25 +0100
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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=3Dserial0,path=3D/tmp/serial0,server,nowait
>> (qemu) device_add virtserialport,bus=3Dvirtio-serial0.0,nr=3D2,\
>>                   chardev=3Dserial0,id=3Dserial0,name=3Dserial0
>> (qemu) device_del serial0
>> (qemu) device_add virtserialport,bus=3Dvirtio-serial0.0,nr=3D2,\
>>                   chardev=3Dserial0,id=3Dserial0,name=3Dserial0
>> 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 i=
s
>> 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 buff=
ers
>> 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 t=
he
>> buffers if the in_vq has available free slots.
>>
>> Fixes: a7a69ec0d8e4 ("virtio_console: free buffers after reset")
>> Cc: mst@redhat.com
>> Signed-off-by: Laurent Vivier <lvivier@redhat.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)
>>  =09spin_lock_init(&port->outvq_lock);
>>  =09init_waitqueue_head(&port->waitqueue);
>> =20
>> -=09/* Fill the in_vq with buffers so the host can send us data. */
>> -=09nr_added_bufs =3D fill_queue(port->in_vq, &port->inbuf_lock);
>> -=09if (!nr_added_bufs) {
>> -=09=09dev_err(port->dev, "Error allocating inbufs\n");
>> -=09=09err =3D -ENOMEM;
>> -=09=09goto free_device;
>> +=09/* if the in_vq has not already been filled (the port has already be=
en
>> +=09 * used and unplugged), fill the in_vq with buffers so the host can
>> +=09 * send us data.
>> +=09 */
>> +=09if (port->in_vq->num_free !=3D 0) {
>> +=09=09nr_added_bufs =3D fill_queue(port->in_vq, &port->inbuf_lock);
>> +=09=09if (!nr_added_bufs) {
>> +=09=09=09dev_err(port->dev, "Error allocating inbufs\n");
>> +=09=09=09err =3D -ENOMEM;
>> +=09=09=09goto free_device;
>> +=09=09}
>>  =09}
>> =20
>>  =09if (is_rproc_serial(port->portdev->vdev))
>=20
> 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