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Subject: Re: [PATCH] virtio: Honour 'may_reduce_num' in vring_create_virtqueue
To:     Cornelia Huck <cohuck@redhat.com>,
        "Michael S . Tsirkin" <mst@redhat.com>
Cc:     Halil Pasic <pasic@linux.ibm.com>,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
References: <20190408123322.24086-1-cohuck@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <9b3d1789-33e1-3ad5-18cd-cacefec8bbb2@redhat.com>
Date:   Tue, 9 Apr 2019 17:00:59 +0800
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On 2019/4/8 下午8:33, Cornelia Huck wrote:
> vring_create_virtqueue() allows the caller to specify via the
> may_reduce_num parameter whether the vring code is allowed to
> allocate a smaller ring than specified.
>
> However, the split ring allocation code tries to allocate a
> smaller ring on allocation failure regardless of what the
> caller specified. This may cause trouble for e.g. virtio-pci
> in legacy mode, which does not support ring resizing. (The
> packed ring code does not resize in any case.)
>
> Let's fix this by bailing out immediately in the split ring code
> if the requested size cannot be allocated and may_reduce_num has
> not been specified.
>
> While at it, fix a typo in the usage instructions.
>
> Fixes: 2a2d1382fe9d ("virtio: Add improved queue allocation API")
> Cc: stable@vger.kernel.org # v4.6+
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
>   drivers/virtio/virtio_ring.c | 2 ++
>   include/linux/virtio_ring.h  | 2 +-
>   2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 18846afb39da..5df92c308286 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -882,6 +882,8 @@ static struct virtqueue *vring_create_virtqueue_split(
>   					  GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
>   		if (queue)
>   			break;
> +		if (!may_reduce_num)
> +			return NULL;
>   	}
>   
>   	if (!num)
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index fab02133a919..3dc70adfe5f5 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -63,7 +63,7 @@ struct virtqueue;
>   /*
>    * Creates a virtqueue and allocates the descriptor ring.  If
>    * may_reduce_num is set, then this may allocate a smaller ring than
> - * expected.  The caller should query virtqueue_get_ring_size to learn
> + * expected.  The caller should query virtqueue_get_vring_size to learn
>    * the actual size of the ring.
>    */
>   struct virtqueue *vring_create_virtqueue(unsigned int index,


Acked-by: Jason Wang <jasowang@redhat.com>