KVM: s390: Fix for 5.8

Hi Paolo,

please pull this fix reducing the number of (currently unused) iopins for kvm/master.

The following changes since commit b3a9e3b9622ae10064826dccb4f7a52bd88c7407:

Linux 5.8-rc1 (2020-06-14 12:45:04 -0700)

are available in the Git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git tags/kvm-s390-master-5.8-3

for you to fetch changes up to 774911290c589e98e3638e73b24b0a4d4530e97c:

KVM: s390: reduce number of IO pins to 1 (2020-06-18 09:48:19 +0200)

----------------------------------------------------------------
The current number of KVM_IRQCHIP_NUM_PINS results in an order 3 allocation (32kb) for each guest start/restart which can result in OOM killer activity when kernel memory is fragmented enough.

This fix reduces the number of iopins as s390 doesn't use them, hence reducing the memory footprint.

----------------------------------------------------------------

Christian Borntraeger (1):
KVM: s390: reduce number of IO pins to 1

 arch/s390/include/asm/kvm_host.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

2.25.4

From: Christian Borntraeger <[email protected]>

The current number of KVM_IRQCHIP_NUM_PINS results in an order 3 allocation (32kb) for each guest start/restart. This can result in OOM killer activity even with free swap when the memory is fragmented enough:

kernel: qemu-system-s39 invoked oom-killer: gfp_mask=0x440dc0(GFP_KERNEL_ACCOUNT|__GFP_COMP|__GFP_ZERO), order=3, oom_score_adj=0
kernel: CPU: 1 PID: 357274 Comm: qemu-system-s39 Kdump: loaded Not tainted 5.4.0-29-generic #33-Ubuntu
kernel: Hardware name: IBM 8562 T02 Z06 (LPAR)
kernel: Call Trace:
kernel: ([<00000001f848fe2a>] show_stack+0x7a/0xc0)
kernel:  [<00000001f8d3437a>] dump_stack+0x8a/0xc0
kernel:  [<00000001f8687032>] dump_header+0x62/0x258
kernel:  [<00000001f8686122>] oom_kill_process+0x172/0x180
kernel:  [<00000001f8686abe>] out_of_memory+0xee/0x580
kernel:  [<00000001f86e66b8>] __alloc_pages_slowpath+0xd18/0xe90
kernel:  [<00000001f86e6ad4>] __alloc_pages_nodemask+0x2a4/0x320
kernel:  [<00000001f86b1ab4>] kmalloc_order+0x34/0xb0
kernel:  [<00000001f86b1b62>] kmalloc_order_trace+0x32/0xe0
kernel:  [<00000001f84bb806>] kvm_set_irq_routing+0xa6/0x2e0
kernel:  [<00000001f84c99a4>] kvm_arch_vm_ioctl+0x544/0x9e0
kernel:  [<00000001f84b8936>] kvm_vm_ioctl+0x396/0x760
kernel:  [<00000001f875df66>] do_vfs_ioctl+0x376/0x690
kernel:  [<00000001f875e304>] ksys_ioctl+0x84/0xb0
kernel:  [<00000001f875e39a>] __s390x_sys_ioctl+0x2a/0x40
kernel:  [<00000001f8d55424>] system_call+0xd8/0x2c8

As far as I can tell s390x does not use the iopins as we bail our for anything other than KVM_IRQ_ROUTING_S390_ADAPTER and the chip/pin is only used for KVM_IRQ_ROUTING_IRQCHIP. So let us use a small number to reduce the memory footprint.

---
 arch/s390/include/asm/kvm_host.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index cee3cb6455a2..6ea0820e7c7f 100644
@@ -31,12 +31,12 @@
 #define KVM_USER_MEM_SLOTS 32
 
 /*
- * These seem to be used for allocating ->chip in the routing table,
- * which we don't use. 4096 is an out-of-thin-air value. If we need
- * to look at ->chip later on, we'll need to revisit this.
+ * These seem to be used for allocating ->chip in the routing table, which we
+ * don't use. 1 is as small as we can get to reduce the needed memory. If we
+ * need to look at ->chip later on, we'll need to revisit this.
  */
 #define KVM_NR_IRQCHIPS 1
-#define KVM_IRQCHIP_NUM_PINS 4096
+#define KVM_IRQCHIP_NUM_PINS 1
 #define KVM_HALT_POLL_NS_DEFAULT 50000
 
 /* s390-specific vcpu->requests bit members */

2.25.4

Pulled, thanks. Unfortunately this missed my 5.8-rc3 pull request by a day, but at least it's safe and sound in kvm/master.

Paolo