Re: [PATCH] vsock/virtio: use GFP_ATOMIC under RCU read lock

[Stefano Garzarella]

On Wed, Oct 02, 2024 at 09:41:42AM GMT, Michael S. Tsirkin wrote:

virtio_transport_send_pkt in now called on transport fast path,
under RCU read lock. In that case, we have a bug: virtio_add_sgs
is called with GFP_KERNEL, and might sleep.

Pass the gfp flags as an argument, and use GFP_ATOMIC on
the fast path.

Link: 
https://lore.kernel.org/all/hfcr2aget2zojmqpr4uhlzvnep4vgskblx5b6xf2ddosbsrke7@nt34bxgp7j2x
Fixes: efcd71af38be ("vsock/virtio: avoid queuing packets when intermediate queue is 
empty")
Reported-by: Christian Brauner <brau...@kernel.org>
Cc: Stefano Garzarella <sgarz...@redhat.com>
Cc: Luigi Leonardi <luigi.leona...@outlook.com>
Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
---

Lightly tested. Christian, could you pls confirm this fixes the problem
for you? Stefano, it's a holiday here - could you pls help test!

Sure, thanks for the quick fix! I was thinking something similar ;-)

Thanks!


net/vmw_vsock/virtio_transport.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index f992f9a216f0..0cd965f24609 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -96,7 +96,7 @@ static u32 virtio_transport_get_local_cid(void)

/* Caller need to hold vsock->tx_lock on vq */
static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
-                                    struct virtio_vsock *vsock)
+                                    struct virtio_vsock *vsock, gfp_t gfp)
{
        int ret, in_sg = 0, out_sg = 0;
        struct scatterlist **sgs;
@@ -140,7 +140,7 @@ static int virtio_transport_send_skb(struct sk_buff *skb, 
struct virtqueue *vq,
                }
        }

-       ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
+       ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, gfp);
        /* Usually this means that there is no more space available in
         * the vq
         */
@@ -178,7 +178,7 @@ virtio_transport_send_pkt_work(struct work_struct *work)

                reply = virtio_vsock_skb_reply(skb);

-               ret = virtio_transport_send_skb(skb, vq, vsock);
+               ret = virtio_transport_send_skb(skb, vq, vsock, GFP_KERNEL);
                if (ret < 0) {
                        virtio_vsock_skb_queue_head(&vsock->send_pkt_queue, 
skb);
                        break;
@@ -221,7 +221,7 @@ static int virtio_transport_send_skb_fast_path(struct 
virtio_vsock *vsock, struc
        if (unlikely(ret == 0))
                return -EBUSY;

-       ret = virtio_transport_send_skb(skb, vq, vsock);

nit: maybe we can add a comment here:
        /* GFP_ATOMIC because we are in RCU section, so we can't sleep */

+       ret = virtio_transport_send_skb(skb, vq, vsock, GFP_ATOMIC);
        if (ret == 0)
                virtqueue_kick(vq);

--
MST

I'll run some tests and come back with R-b when it's done.

Thanks,
Stefano