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

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Date:   Thu, 29 Sep 2022 09:40:10 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Michael S. Tsirkin" <mst@...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 04:02:12PM -0400, Michael S. Tsirkin wrote:
>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.

Yes, but we still might not have as many continuous pages to allocate, 
so I would use kvmalloc the same.

I agree that we should do better, I hope that moving to sk_buff will 
allow us to better manage allocation. Maybe after we merge that part we 
should spend some time to solve these problems.

Thanks,
Stefano