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        <uekawa@google.com>
Date:   Thu, 29 Sep 2022 08:14:24 +0900
Message-ID: <CADgJSGHxPWXJjbakEeWnqF42A03yK7Dpw6U1SKNLhk+B248Ymg@mail.gmail.com>
Subject: Re: [PATCH] vhost/vsock: Use kvmalloc/kvfree for larger packets.
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        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,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        Bobby Eshleman <bobby.eshleman@gmail.com>
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2022=E5=B9=B49=E6=9C=8829=E6=97=A5(=E6=9C=A8) 0:11 Stefano Garzarella <sgar=
zare@redhat.com>:
>
> 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@chromium.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?
>
> 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.
>

Thank you for the review! I was wondering if this is a reasonable workaroun=
d (as
we found that this patch makes a reliably crashing system into a
reliably surviving system.)


... Sounds like it is a reasonable patch to use backported to older kernels=
?

> >
> >
> >
> >> > ---
> >> >
> >> > 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 =3D kmalloc(pkt->len, GFP_KERNEL);
> >> > +  pkt->buf =3D 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_b=
uff
> >> [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 us=
ed in
> >> the host, while in the virtio-vsock driver the buffer is exposed to th=
e
> >> 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
>


--=20
Junichi Uekawa
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