linux-kernel - Re: [PATCH] vhost/vsock: Use kvmalloc/kvfree for larger packets.

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Message-ID: <20220928160116-mutt-send-email-mst@kernel.org>
Date:   Wed, 28 Sep 2022 16:02:12 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     Junichi Uekawa <uekawa@...omium.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>, davem@...emloft.net,
        netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org,
        Bobby Eshleman <bobby.eshleman@...il.com>
Subject: Re: [PATCH] vhost/vsock: Use kvmalloc/kvfree for larger packets.

On Wed, Sep 28, 2022 at 05:11:35PM +0200, Stefano Garzarella wrote:
> On Wed, Sep 28, 2022 at 05:31:58AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Sep 28, 2022 at 10:28:23AM +0200, Stefano Garzarella wrote:
> > > 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@...omium.org>
> > 
> > My worry here is that this in more of a work around.
> > It would be better to not allocate memory so aggressively:
> > if we are so short on memory we should probably process
> > packets one at a time. Is that very hard to implement?
> 
> Currently the "virtio_vsock_pkt" is allocated in the "handle_kick" callback
> of TX virtqueue. Then the packet is multiplexed on the right socket queue,
> then the user space can de-queue it whenever they want.
> 
> So maybe we can stop processing the virtqueue if we are short on memory, but
> when can we restart the TX virtqueue processing?

Assuming you added at least one buffer, the time to restart would be
after that buffer has been used.


> I think as long as the guest used only 4K buffers we had no problem, but now
> that it can create larger buffers the host may not be able to allocate it
> contiguously. Since there is no need to have them contiguous here, I think
> this patch is okay.
> 
> However, if we switch to sk_buff (as Bobby is already doing), maybe we don't
> have this problem because I think there is some kind of pre-allocated pool.
> 
> > 
> > 
> > 
> > > > ---
> > > >
> > > > 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()).
> > 
> > More importantly it needs to support DMA API which IIUC kvmalloc
> > memory does not.
> > 
> 
> Right, good point!
> 
> Thanks,
> Stefano