test-runner: fix OOM issues (hopefully)

For quite a while test-runner has run into frequent OOM exceptions when
running many tests in a row. Its not completely known exactly why, but
seems to point to the 9p driver which is used for sharing the root fs
between the test-runner VM and the host.

With debugging enabled (-d) one can see the available memory available
relatively stable. If a test fails it may spike ~3-4kb but this quickly
recovers as python garbage collects.

At some point the kernel faults failing to allocate which (usually) is
shown by a python OOM exception. At this point there is plenty of
available memory.

Dumping the kernel trace its seen that the 9p driver is involved:

[  248.962949] test-runner: page allocation failure: order:7, mode:0x40dc0(GFP_KERNEL|__GFP_COMP|__GFP_ZERO), nodemask=(null),cpuset=/,mems_allowed=0
[  248.962958] CPU: 2 PID: 477 Comm: test-runner Not tainted 5.16.0 #91
[  248.962960] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-4.fc34 04/01/2014
[  248.962961] Call Trace:
[  248.962964]  <TASK>
[  248.962965]  dump_stack_lvl+0x34/0x44
[  248.962971]  warn_alloc.cold+0x78/0xdc
[  248.962975]  ? __alloc_pages_direct_compact+0x14c/0x1e0
[  248.962979]  __alloc_pages_slowpath.constprop.0+0xbfe/0xc60
[  248.962982]  __alloc_pages+0x2d5/0x2f0
[  248.962984]  kmalloc_order+0x23/0x80
[  248.962988]  kmalloc_order_trace+0x14/0x80
[  248.962990]  v9fs_alloc_rdir_buf.isra.0+0x1f/0x30
[  248.962994]  v9fs_dir_readdir+0x51/0x1d0
[  248.962996]  ? __handle_mm_fault+0x6e0/0xb40
[  248.962999]  ? inode_security+0x1d/0x50
[  248.963009]  ? selinux_file_permission+0xff/0x140
[  248.963011]  iterate_dir+0x16f/0x1c0
[  248.963014]  __x64_sys_getdents64+0x7b/0x120
[  248.963016]  ? compat_fillonedir+0x150/0x150
[  248.963019]  do_syscall_64+0x3b/0x90
[  248.963021]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  248.963024] RIP: 0033:0x7fedd7c6d8c7
[  248.963026] Code: 00 00 0f 05 eb b7 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa b8 ff ff ff 7f 48 39 c2 48 0f 47 d0 b8 d9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 8b 15 81 a5 0f 00 f7 d8 64 89 02 48
[  248.963028] RSP: 002b:00007ffd06cd87e8 EFLAGS: 00000293 ORIG_RAX: 00000000000000d9
[  248.963031] RAX: ffffffffffffffda RBX: 000056090d87dd20 RCX: 00007fedd7c6d8c7
[  248.963032] RDX: 0000000000080000 RSI: 000056090d87dd50 RDI: 000000000000000f
[  248.963033] RBP: 000056090d87dd50 R08: 0000000000000030 R09: 00007fedc7d37af0
[  248.963035] R10: 00007fedc7d7d730 R11: 0000000000000293 R12: ffffffffffffff88
[  248.963038] R13: 000056090d87dd24 R14: 0000000000000000 R15: 000056090d0485e8

Here its seen an allocation of 512k is being requested (order:7), but faults.
In this run it there was ~35MB of available memory on the system.

Available Memory: 35268 kB
Last Test Delta: -2624 kB
Per-test Usage:
[  0] **        			37016
[  1] ********* 			41584
[  2] *         			36280
[  3] ********* 			41452
[  4] ********  			40940
[  5] ******    			39284
[  6] ****      			38348
[  7] ***       			37496
[  8] ****      			37892
[  9]           			35268

This can be reproduced by running all autotests (changing the ram down to
~128MB helps trigger it faster):

./tools/test-runner -k <kernel> -d

After many attempts to fix this it was finally found that simply removing the
explicit 9p2000.u version from the kernel command line 'fixed' the problem.
This even allows decreasing the RAM down to 256MB from 384MB and so far no
OOM's have been seen.

tools/test-runner

@@ -1244,6 +1244,7 @@ def start_test(ctx, subtests, rqueue):
 		try:
 			module = importlib.import_module(os.path.splitext(s)[0])
 		except OSError as e:
+			dbg(subprocess.check_output("cat /proc/buddyinfo", shell=True).decode('utf-8'))
 			dbg(subprocess.check_output("dmesg | tail -80", shell=True).decode('utf-8'))
 			print(ctx)
 			raise e
@@ -1891,13 +1892,7 @@ class Main:
 		else:
 			smp = int(nproc / 2)
 
-		#
-		# Increase RAM if valgrind is being used
-		#
-		if self.args.valgrind:
-			ram = 512
-		else:
-			ram = 384
+		ram = 256
 
 		print("Using %d cores, %d RAM for VM" % (smp, ram))
 
@@ -1936,7 +1931,7 @@ class Main:
 			'-append',
 			'console=ttyS0,115200n8 earlyprintk=serial \
 				rootfstype=9p root=/dev/root \
-				rootflags=trans=virtio,msize=1048576,version=9p2000.u \
+				rootflags=trans=virtio \
 				acpi=off pci=noacpi %s ro \
 				mac80211_hwsim.radios=0 %s' % (kern_log, options),
 			'-smp', str(smp)