From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36003) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f999P-0003F5-07 for qemu-devel@nongnu.org; Thu, 19 Apr 2018 08:58:59 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f999L-0001YH-TH for qemu-devel@nongnu.org; Thu, 19 Apr 2018 08:58:59 -0400 Date: Thu, 19 Apr 2018 14:58:40 +0200 From: Cornelia Huck Message-ID: <20180419145840.324602ff.cohuck@redhat.com> In-Reply-To: <20180419143318.4e24edaf@redhat.com> References: <20180419072123.682-1-david@gibson.dropbear.id.au> <20180419143318.4e24edaf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [qemu-s390x] [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: Igor Mammedov , Christian Borntraeger Cc: David Gibson , ehabkost@redhat.com, David Hildenbrand , groug@kaod.org, qemu-devel@nongnu.org, qemu-s390x@nongnu.org, qemu-ppc@nongnu.org, clg@kaod.org, pbonzini@redhat.com On Thu, 19 Apr 2018 14:33:18 +0200 Igor Mammedov wrote: > On Thu, 19 Apr 2018 17:21:23 +1000 > David Gibson 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 > > --- > > 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? I don't quite remember the cmm semantics here -- Christian?