Re: [Qemu-devel] [qemu-s390x] [PATCH for-2.13] Clear mem_path if we fall back to anonymous RAM allocation

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Apr 19, 2018 at 06:08:51PM +0200, Greg Kurz wrote:
> On Thu, 19 Apr 2018 16:11:37 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
> > On 19.04.2018 15:34, Christian Borntraeger wrote:
> > > 
> > > 
> > > On 04/19/2018 02:58 PM, Cornelia Huck wrote:  
> > >> On Thu, 19 Apr 2018 14:33:18 +0200
> > >> Igor Mammedov <imammedo@redhat.com> wrote:
> > >>  
> > >>> On Thu, 19 Apr 2018 17:21:23 +1000
> > >>> David Gibson <david@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 <david@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
> 
> Something similar happens with spapr: kvm_fixup_page_sizes() calls
> qemu_getrampagesize() during CPU start, which happens before the machine
> init calls allocate_system_memory_nonnuma(). Shouldn't we allocate memory
> before calling spapr_init_cpus() in spapr_machine_init() then ?

Note that the way kvm_fixup_page_sizes() works is broken in it's own
right - this patch was actually written as a prliminary to fixing
that.

> > >>> 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?  
> > > 
> > > The CMMA interface does not work on large pages. I think the kernel will react
> > > with EFAULT in some cases (cmma migration and others) so qemu will probably fail
> > > unexpectedly. 
> > > 
> > > But this patch seems to only clear mem-path if we do not allocate at all from
> > > hugetlbfs. So things should be ok, no?
> > > 
> > >   
> > 
> > This even looks like the right thing to me, as hugetlbfs was never
> > supported.
> > 
> 
> Unrelated to this patch, -mem-path can be passed something that doesn't sit
> in a hugetlbfs, in which case we use getpagesize()... is there a reason for
> kvm_s390_enable_cmma() to filter out this case as well ? Or should we rather
> check mem_path isn't NULL and points to a hugetlbfs ?
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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