[PATCH] hostmem: don't use mbind() if host-nodes is epmty

[Igor Mammedov <imammedo@redhat.com>]

Since 5.0 QEMU uses hostmem backend for allocating main guest RAM.
The backend however calls mbind() which is typically NOP
in case of default policy/absent host-nodes bitmap.
However when runing in container with black-listed mbind()
syscall, QEMU fails to start with error
 "cannot bind memory to host NUMA nodes: Operation not permitted"
even when user hasn't provided host-nodes to pin to explictly
(which is the case with -m option)

To fix issue, call mbind() only in case when user has provided
host-nodes explicitly (i.e. host_nodes bitmap is not empty).
That should allow to run QEMU in containers with black-listed
mbind() without memory pinning. If QEMU provided memory-pinning
is required user still has to white-list mbind() in container
configuration.

Reported-by: Manuel Hohmann <mhohmann@physnet.uni-hamburg.de>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
CC: berrange@redhat.com
CC: ehabkost@redhat.com
CC: pbonzini@redhat.com
CC: mhohmann@physnet.uni-hamburg.de
CC: qemu-stable@nongnu.org
---
 backends/hostmem.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/backends/hostmem.c b/backends/hostmem.c
index 327f9eebc3..0efd7b7bd6 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -383,8 +383,10 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
         assert(sizeof(backend->host_nodes) >=
                BITS_TO_LONGS(MAX_NODES + 1) * sizeof(unsigned long));
         assert(maxnode <= MAX_NODES);
-        if (mbind(ptr, sz, backend->policy,
-                  maxnode ? backend->host_nodes : NULL, maxnode + 1, flags)) {
+
+        if (maxnode &&
+            mbind(ptr, sz, backend->policy, backend->host_nodes, maxnode + 1,
+                  flags)) {
             if (backend->policy != MPOL_DEFAULT || errno != ENOSYS) {
                 error_setg_errno(errp, errno,
                                  "cannot bind memory to host NUMA nodes");
-- 
2.18.1