From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56766) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f93sp-00014l-4L for qemu-devel@nongnu.org; Thu, 19 Apr 2018 03:21:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f93so-00027M-3v for qemu-devel@nongnu.org; Thu, 19 Apr 2018 03:21:31 -0400 From: David Gibson Date: Thu, 19 Apr 2018 17:21:23 +1000 Message-Id: <20180419072123.682-1-david@gibson.dropbear.id.au> Subject: [Qemu-devel] [PATCH for-2.13] Clear mem_path if we fall back to anonymous RAM allocation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: pbonzini@redhat.com, imammedo@redhat.com, ehabkost@redhat.com Cc: groug@kaod.org, clg@kaod.org, qemu-ppc@nongnu.org, qemu-devel@nongnu.org, David Gibson 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 --- 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 -- 2.14.3