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

[Hu Tao]

On Thu, Aug 14, 2014 at 02:31:41PM +0800, zhanghailiang wrote:
> On 2014/8/13 19:50, Michael S. Tsirkin wrote:
> >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<address@hidden>
> >>Signed-off-by: zhanghailiang<address@hidden>
> >
> >OK but won't this still fail in case of memory hotplug?
> >We set MCL_FUTURE so the same will apply?

It has already been set.

> >Maybe it's enough to set MPOL_MF_MOVE?
> >Does the following work for you?
> >
> 
> Hi Michael,
> 
> I have tested memory hotplug, use virsh command like
> 'virsh setmem redhat-6.4 6388608 --config --live', it is OK, and
> it will not call mbind when do such memory hotplug.
> but i don't know if there is command like 'memory-node hotplug' ?

Using qemu monitor command:
object_add memory-backend-ram,id=ram1,size=128M,host-nodes=0,policy=bind

> 
> MPOL_MF_MOVE can work, it is more simple, but it is not perfect. It
> consumes more time to *move the memory*(i guess will reconstruct
> pages and copy memory)which has been locked by mlockall. The result
> is VM will start slower than the above scenario.

I think your patch makes sense but it doesn't work for hotplugged
memory. We need MPOL_MF_MOVE, too.

> 
> BTW, i think the follow process is clearer and more logical:
> Allocate memory--->Set memory policy--->Lock memory.
> 
> So what's your opinion? Thanks very much.
> 
> 
> >-->
> >
> >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<address@hidden>
> >
> >---
> >
> >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(). */
> >
>