Date:   Thu, 29 Sep 2022 03:19:14 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Junichi Uekawa (上川純一) 
        <uekawa@...gle.com>
Cc:     Stefano Garzarella <sgarzare@...hat.com>,
        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 Thu, Sep 29, 2022 at 08:14:24AM +0900, Junichi Uekawa (上川純一) wrote:
> 2022年9月29日(木) 0:11 Stefano Garzarella <sgarzare@...hat.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@...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?
> >
> > 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 workaround (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?

Hmm. Good point about stable. OK.

Acked-by: Michael S. Tsirkin <mst@...hat.com>


> > >
> > >
> > >
> > >> > ---
> > >> >
> > >> > 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
> >
> 
> 
> -- 
> Junichi Uekawa
> Google

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