add workaround kernel patch for vsock panics

Allocation failures for vsock packet buffers occur routinely when
downloading more than one stream at the same time, with less then 512
MiB of RAM it sometimes even occurs for single downloads.

This appears to fix it in all of my reproducer scenarios, tested with up
to 6 downloads at once in a 128 MiB RAM machine.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

diff --git a/src/patches/kernel/0003-vsock-reduce-packet-size.patch b/src/patches/kernel/0003-vsock-reduce-packet-size.patch
new file mode 100644 (file)
index 0000000..378da53
--- /dev/null
+++ b/src/patches/kernel/0003-vsock-reduce-packet-size.patch
@@ -0,0 +1,36 @@
+From a437d428733881f408b5d42eb75812600083cb75 Mon Sep 17 00:00:00 2001
+From: Stefan Reiter <s.reiter@proxmox.com>
+Date: Mon, 26 Apr 2021 14:08:36 +0200
+Subject: [PATCH] vsock: reduce packet size
+
+Reduce the maximum packet size to avoid allocation errors in VMs with
+very little memory available (since the buffer needs a contiguous
+block of memory, which can get rare for 64kB blocks).
+
+4kB used to be the default, and according to [0] increasing it makes
+the difference between ~25Gb/s and ~40Gb/s - certainly a lot faster,
+but both within the realm of unreachable for our restore scenario.
+
+[0] https://stefano-garzarella.github.io/posts/2019-11-08-kvmforum-2019-vsock/
+
+Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
+---
+ include/linux/virtio_vsock.h | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
+index dc636b727179..18c09ff72929 100644
+--- a/include/linux/virtio_vsock.h
++++ b/include/linux/virtio_vsock.h
+@@ -9,7 +9,7 @@
+ 
+ #define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE      (1024 * 4)
+ #define VIRTIO_VSOCK_MAX_BUF_SIZE             0xFFFFFFFFUL
+-#define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE         (1024 * 64)
++#define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE         (1024 * 4)
+ 
+ enum {
+       VSOCK_VQ_RX     = 0, /* for host to guest data */
+-- 
+2.20.1
+