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

[Greg Kurz <groug@kaod.org>]

On Fri, 15 Jul 2016 10:10:25 +0200
Thomas Huth <thuth@redhat.com> wrote:

> 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

According to http://patchwork.ozlabs.org/patch/584741/ , it is best worded

"Broken in commit 86b50f2e1bef"

> 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) {

Dumb question: why only checking numa_info[0] ?

>          static bool warned;
>          if (!warned) {
>              error_report("Huge page support disabled (n/a for main memory).");