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

[David Hildenbrand <david@redhat.com>]

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
>>> 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.

-- 

Thanks,

David / dhildenb