Re: [Qemu-devel] [PATCH] mlock: fix bug when mlockall called before mbind

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 13, 2014 at 07:21:57PM +0800, zhanghailiang wrote:
> If we configure qemu with realtime-mlock-on and memory-node-bind at the same time,
> Qemu will fail to start, and mbind() fails with message "Input/output error".
> 
> >From man page:
> int mbind(void *addr, unsigned long len, int mode,
>                  unsigned long *nodemask, unsigned long maxnode,
>                  unsigned flags);
> The *MPOL_BIND* mode specifies a strict policy that restricts memory allocation
> to the nodes specified in nodemask.
> If *MPOL_MF_STRICT* is passed in flags and policy is not MPOL_DEFAULT(In qemu
> here is MPOL_BIND), then the call will fail with the error EIO if the existing
> pages in  the memory range don't follow the policy.
> 
> The memory locked ahead by mlockall can not guarantee to follow the policy above,
> And if that happens, it will result in an EIO error.
> 
> So we should call mlock after mbind, here we adjust the place where called mlock,
> Move it to function pc_memory_init.
> 
> Signed-off-by: xiexiangyou <xiexiangyou@huawei.com>
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>

OK but won't this still fail in case of memory hotplug?
We set MCL_FUTURE so the same will apply?
Maybe it's enough to set MPOL_MF_MOVE?
Does the following work for you?

-->

hostmem: set MPOL_MF_MOVE

When memory is allocated on a wrong node, MPOL_MF_STRICT
doesn't move it - it just fails the allocation.
A simple way to reproduce the failure is with mlock=on
realtime feature.

The code comment actually says: "ensure policy won't be ignored"
so setting MPOL_MF_MOVE seems like a better way to do this.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---

diff --git a/backends/hostmem.c b/backends/hostmem.c
index ca10c51..a9905c0 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -304,7 +304,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
         /* ensure policy won't be ignored in case memory is preallocated
          * before mbind(). note: MPOL_MF_STRICT is ignored on hugepages so
          * this doesn't catch hugepage case. */
-        unsigned flags = MPOL_MF_STRICT;
+        unsigned flags = MPOL_MF_STRICT | MPOL_MF_MOVE;
 
         /* check for invalid host-nodes and policies and give more verbose
          * error messages than mbind(). */

-- 
MST