Re: [PATCH v2 hci-7.4] hostmem: fix live update crash of VMs with bound guest memory #VSTOR-147016

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

LGTM
Ack

On 9/23/26 8:31 PM, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> A live QEMU update kills a VM whose memory backend has host-nodes when
> the emulator cgroup no longer allows those nodes. The old QEMU saves its
> state, the new one exits during startup with
> 
>   qemu-kvm: cannot bind memory to host NUMA nodes: Invalid argument
> 
> and the guest is lost.
> 
> Guest RAM survives vz-cprsave restart as the same anonymous mapping,
> carrying its pages and its memory policy.
> host_memory_backend_memory_complete() does not know that and treats it
> as fresh memory: it calls mbind() with MPOL_MF_STRICT | MPOL_MF_MOVE and
> preallocates again. The exec keeps the PID and the cgroup, so if
> cpuset.mems has been changed to nodes outside host-nodes since the VM
> started, mbind() returns EINVAL and QEMU exits. Where it succeeds, it
> only walks or migrates pages that are already in place, while the guest
> is stopped.
> 
> Skip the NUMA binding and preallocation for an anonymous backend while
> the new process is being restored. Test CPR_RESTART_POSTEXEC rather than
> cpr_restart_in_progress(): the latter also covers CPR_RESTART_PREEXEC,
> which a failed vz-cprsave leaves set in the old process, and memory
> hotplugged there afterwards would lose its binding. Backends with an fd
> get a new mapping after the exec and keep mbind().
> 
> Suggested-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> ---
>  backends/hostmem.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/backends/hostmem.c b/backends/hostmem.c
> index 747e7838c03..5f36d7aa938 100644
> --- a/backends/hostmem.c
> +++ b/backends/hostmem.c
> @@ -20,6 +20,7 @@
>  #include "qom/object_interfaces.h"
>  #include "qemu/mmap-alloc.h"
>  #include "qemu/madvise.h"
> +#include "qemu/cpr-env.h"
>  
>  #ifdef CONFIG_NUMA
>  #include <numaif.h>
> @@ -343,6 +344,11 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
>          if (!backend->dump) {
>              qemu_madvise(ptr, sz, QEMU_MADV_DONTDUMP);
>          }
> +        /* Anonymous memory is inherited on restart with its policy and pages */
> +        if (cpr_getenv_bool("CPR_RESTART_POSTEXEC") &&
> +            memory_region_get_fd(&backend->mr) < 0) {
> +            goto out;
> +        }
>  #ifdef CONFIG_NUMA
>          unsigned long lastbit = find_last_bit(backend->host_nodes, MAX_NODES);
>          /* lastbit == MAX_NODES means maxnode = 0 */
> 
> base-commit: c57b00198350b12ab35649707528304a2d34c0e9