[Qemu-devel] [PATCH] ppc: Yet another fix for the huge page support detection mechanism

[Thomas Huth <thuth@redhat.com>]

Commit 86b50f2e1bef ("Disable huge page support if it is not available
for main RAM") already made sure that huge page support is not announced
to the guest if the normal RAM of non-NUMA configurations is not backed
by a huge page filesystem. However, there is one more case that can go
wrong: NUMA is enabled, but the RAM of the NUMA nodes are not configured
with huge page support (and only the memory of a DIMM is configured with
it). When QEMU is started with the following command line for example,
the Linux guest currently crashes because it is trying to use huge pages
on a memory region that does not support huge pages:

 qemu-system-ppc64 -enable-kvm ... -m 1G,slots=4,maxmem=32G -object \
   memory-backend-file,policy=default,mem-path=/hugepages,size=1G,id=mem-mem1 \
   -device pc-dimm,id=dimm-mem1,memdev=mem-mem1 -smp 2 \
   -numa node,nodeid=0 -numa node,nodeid=1

To fix this issue, we've got to make sure to disable huge page support,
too, when there is a NUMA node that is not using a memory backend with
huge page support.

Fixes: 86b50f2e1befc33407bdfeb6f45f7b0d2439a740
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 target-ppc/kvm.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 884d564..7a8f555 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -389,12 +389,16 @@ static long getrampagesize(void)
 
     object_child_foreach(memdev_root, find_max_supported_pagesize, &hpsize);
 
-    if (hpsize == LONG_MAX) {
+    if (hpsize == LONG_MAX || hpsize == getpagesize()) {
         return getpagesize();
     }
 
-    if (nb_numa_nodes == 0 && hpsize > getpagesize()) {
-        /* No NUMA nodes and normal RAM without -mem-path ==> no huge pages! */
+    /* If NUMA is disabled or the NUMA nodes are not backed with a
+     * memory-backend, then there is at least one node using "normal"
+     * RAM. And since normal RAM has not been configured with "-mem-path"
+     * (what we've checked earlier here already), we can not use huge pages!
+     */
+    if (nb_numa_nodes == 0 || numa_info[0].node_memdev == NULL) {
         static bool warned;
         if (!warned) {
             error_report("Huge page support disabled (n/a for main memory).");
-- 
1.8.3.1