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Date:   Wed, 28 Sep 2022 10:28:23 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Junichi Uekawa <uekawa@chromium.org>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Eric Dumazet <edumazet@google.com>, davem@davemloft.net,
        netdev@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        mst@redhat.com, Paolo Abeni <pabeni@redhat.com>,
        linux-kernel@vger.kernel.org,
        Bobby Eshleman <bobby.eshleman@gmail.com>
Subject: Re: [PATCH] vhost/vsock: Use kvmalloc/kvfree for larger packets.
Message-ID: <20220928082823.wyxplop5wtpuurwo@sgarzare-redhat>
References: <20220928064538.667678-1-uekawa@chromium.org>
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On Wed, Sep 28, 2022 at 03:45:38PM +0900, Junichi Uekawa wrote:
>When copying a large file over sftp over vsock, data size is usually 32kB,
>and kmalloc seems to fail to try to allocate 32 32kB regions.
>
> Call Trace:
>  [<ffffffffb6a0df64>] dump_stack+0x97/0xdb
>  [<ffffffffb68d6aed>] warn_alloc_failed+0x10f/0x138
>  [<ffffffffb68d868a>] ? __alloc_pages_direct_compact+0x38/0xc8
>  [<ffffffffb664619f>] __alloc_pages_nodemask+0x84c/0x90d
>  [<ffffffffb6646e56>] alloc_kmem_pages+0x17/0x19
>  [<ffffffffb6653a26>] kmalloc_order_trace+0x2b/0xdb
>  [<ffffffffb66682f3>] __kmalloc+0x177/0x1f7
>  [<ffffffffb66e0d94>] ? copy_from_iter+0x8d/0x31d
>  [<ffffffffc0689ab7>] vhost_vsock_handle_tx_kick+0x1fa/0x301 [vhost_vsock]
>  [<ffffffffc06828d9>] vhost_worker+0xf7/0x157 [vhost]
>  [<ffffffffb683ddce>] kthread+0xfd/0x105
>  [<ffffffffc06827e2>] ? vhost_dev_set_owner+0x22e/0x22e [vhost]
>  [<ffffffffb683dcd1>] ? flush_kthread_worker+0xf3/0xf3
>  [<ffffffffb6eb332e>] ret_from_fork+0x4e/0x80
>  [<ffffffffb683dcd1>] ? flush_kthread_worker+0xf3/0xf3
>
>Work around by doing kvmalloc instead.
>
>Signed-off-by: Junichi Uekawa <uekawa@chromium.org>
>---
>
> drivers/vhost/vsock.c                   | 2 +-
> net/vmw_vsock/virtio_transport_common.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>index 368330417bde..5703775af129 100644
>--- a/drivers/vhost/vsock.c
>+++ b/drivers/vhost/vsock.c
>@@ -393,7 +393,7 @@ vhost_vsock_alloc_pkt(struct vhost_virtqueue *vq,
> 		return NULL;
> 	}
>
>-	pkt->buf = kmalloc(pkt->len, GFP_KERNEL);
>+	pkt->buf = kvmalloc(pkt->len, GFP_KERNEL);
> 	if (!pkt->buf) {
> 		kfree(pkt);
> 		return NULL;
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index ec2c2afbf0d0..3a12aee33e92 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -1342,7 +1342,7 @@ EXPORT_SYMBOL_GPL(virtio_transport_recv_pkt);
>
> void virtio_transport_free_pkt(struct virtio_vsock_pkt *pkt)
> {
>-	kfree(pkt->buf);
>+	kvfree(pkt->buf);

virtio_transport_free_pkt() is used also in virtio_transport.c and 
vsock_loopback.c where pkt->buf is allocated with kmalloc(), but IIUC 
kvfree() can be used with that memory, so this should be fine.

> 	kfree(pkt);
> }
> EXPORT_SYMBOL_GPL(virtio_transport_free_pkt);
>-- 
>2.37.3.998.g577e59143f-goog
>

This issue should go away with the Bobby's work about introducing 
sk_buff [1], but we can queue this for now.

I'm not sure if we should do the same also in the virtio-vsock driver 
(virtio_transport.c). Here in vhost-vsock the buf allocated is only used 
in the host, while in the virtio-vsock driver the buffer is exposed to 
the device emulated in the host, so it should be physically contiguous 
(if not, maybe we need to adjust virtio_vsock_rx_fill()).
So for now I think is fine to use kvmalloc only on vhost-vsock 
(eventually we can use it also in vsock_loopback), since the Bobby's 
patch should rework this code:

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

[1] 
https://lore.kernel.org/lkml/65d117ddc530d12a6d47fcc45b38891465a90d9f.1660362668.git.bobby.eshleman@bytedance.com/

Thanks,
Stefano