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

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Sep 29, 2022 at 03:19:14AM -0400, Michael S. Tsirkin wrote:
>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@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 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.

Right, so in this case I think is better to add a Fixes tag. Since we 
used kmalloc from the beginning we can use the following:

Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")

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

@Michael are you queueing this, or should it go through net tree?

Thanks,
Stefano