On Thu, 19 Apr 2018 17:21:23 +1000 David Gibson <da...@gibson.dropbear.id.au> wrote:
> If the -mem-path option is set, we attempt to map the guest's RAM from a > file in the given path; it's usually used to back guest RAM with hugepages. > If we're unable to (e.g. not enough free hugepages) then we fall back to > allocating normal anonymous pages. This behaviour can be surprising, but a > comment in allocate_system_memory_nonnuma() suggests it's legacy behaviour > we can't change. > > What really isn't ok, though, is that in this case we leave mem_path set. > That means functions which attempt to determine the pagesize of main RAM > can erroneously think it is hugepage based on the requested path, even > though it's not. > > This is particular bad for the pseries machine type. KVM HV limitations > mean the guest can't use pagesizes larger than the host page size used to > back RAM. That means that such a fallback, rather than merely giving > poorer performance that expected will cause the guest to freeze up early in > boot as it attempts to use large page mappings that can't work. > > This patch addresses the problem by clearing the mem_path variable when we > fall back to anonymous pages, meaning that subsequent attempts to > determine the RAM page size will get an accurate result. > > Signed-off-by: David Gibson <da...@gibson.dropbear.id.au> > --- > numa.c | 1 + > 1 file changed, 1 insertion(+) > > Paolo et al, as with my earlier patches adding some extensions to the > helpers for determining backing page sizes, if there are no objections > can I get an ack to merge this via my ppc tree? > > diff --git a/numa.c b/numa.c > index 1116c90af9..78a869e598 100644 > --- a/numa.c > +++ b/numa.c > @@ -469,6 +469,7 @@ static void allocate_system_memory_nonnuma(MemoryRegion > *mr, Object *owner, > /* Legacy behavior: if allocation failed, fall back to > * regular RAM allocation. > */ > + mem_path = NULL; > memory_region_init_ram_nomigrate(mr, owner, name, ram_size, > &error_fatal); > } > #else mem_path is also used by kvm_s390_apply_cpu_model(), and in ccw_init() memory is initialized before CPUs are so if QEM was started with -mem-path, then before patch created CPU won't have CMM enabled and print warning: "CMM will not be enabled because it is not compatible with hugetlbfs." and after patch it might enable CMM if we clear mem_path. So question is do we care about this? PS: CCing s390 folks.
