Re: [Qemu-devel] [PATCH] ppc: Huge page detection mechanism fixes - Episode III

[Greg Kurz <groug@kaod.org>]

On Mon, 18 Jul 2016 15:19:04 +0200
Thomas Huth <thuth@redhat.com> wrote:

> After already fixing two issues with the huge page detection mechanism
> (see commit 159d2e39a860 and 86b50f2e1bef), Greg Kurz noticed another
> case that caused the guest to crash where QEMU announces huge pages
> though they should not be available for the guest:
> 
> qemu-system-ppc64 -enable-kvm ... -mem-path /dev/hugepages \
>  -m 1G,slots=4,maxmem=32G
>  -object memory-backend-ram,policy=default,size=1G,id=mem-mem1 \
>  -device pc-dimm,id=dimm-mem1,memdev=mem-mem1 -smp 2 \
>  -numa node,nodeid=0 -numa node,nodeid=1
> 
> That means if there is a global mem-path option, we still have
> to look at the memory-backend objects that have been specified
> additionally and return their minimum page size if that value
> is smaller than the page size of the main memory.
> 
> Reported-by: Greg Kurz <groug@kaod.org>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---

Just one remark, see below, but apart from that:

Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>

>  target-ppc/kvm.c | 27 ++++++++++++++-------------
>  1 file changed, 14 insertions(+), 13 deletions(-)
> 
> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
> index 7a8f555..97ab450 100644
> --- a/target-ppc/kvm.c
> +++ b/target-ppc/kvm.c
> @@ -366,10 +366,13 @@ static int find_max_supported_pagesize(Object *obj, void *opaque)
>  static long getrampagesize(void)
>  {
>      long hpsize = LONG_MAX;
> +    long mainrampagesize;
>      Object *memdev_root;
>  
>      if (mem_path) {
> -        return gethugepagesize(mem_path);
> +        mainrampagesize = gethugepagesize(mem_path);
> +    } else {
> +        mainrampagesize = getpagesize();
>      }
>  
>      /* it's possible we have memory-backend objects with
> @@ -383,28 +386,26 @@ static long getrampagesize(void)
>       * backend isn't backed by hugepages.
>       */
>      memdev_root = object_resolve_path("/objects", NULL);
> -    if (!memdev_root) {
> -        return getpagesize();
> +    if (memdev_root) {
> +        object_child_foreach(memdev_root, find_max_supported_pagesize, &hpsize);
>      }
> -
> -    object_child_foreach(memdev_root, find_max_supported_pagesize, &hpsize);
> -
> -    if (hpsize == LONG_MAX || hpsize == getpagesize()) {
> -        return getpagesize();
> +    if (hpsize == LONG_MAX) {
> +        /* No additional memory regions found ==> Report main RAM page size */
> +        return mainrampagesize;
>      }
>  
>      /* 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!
> +     * memory-backend, then there is at least one node using "normal" RAM,
> +     * so if its page size is smaller we have got to report that size instead.
>       */
> -    if (nb_numa_nodes == 0 || numa_info[0].node_memdev == NULL) {
> +    if (hpsize > mainrampagesize &&
> +        (nb_numa_nodes == 0 || numa_info[0].node_memdev == NULL)) {
>          static bool warned;
>          if (!warned) {
>              error_report("Huge page support disabled (n/a for main memory).");

Maybe update the error message since we have another condition ?

Something like:

"Huge page support disabled (at least one numa uses standard page size)"

>              warned = true;
>          }
> -        return getpagesize();
> +        return mainrampagesize;
>      }
>  
>      return hpsize;