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Re: [Qemu-ppc] [qemu-s390x] [Qemu-devel] [PATCH for-2.13] Clear mem_path
Re: [Qemu-ppc] [qemu-s390x] [Qemu-devel] [PATCH for-2.13] Clear mem_path if we fall back to anonymous RAM allocation
Thu, 19 Apr 2018 16:11:37 +0200
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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 <address@hidden> wrote:
>>> On Thu, 19 Apr 2018 17:21:23 +1000
>>> David Gibson <address@hidden> 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 <address@hidden>
>>>> 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,
>>> 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
> 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
David / dhildenb
Re: [Qemu-ppc] [PATCH for-2.13] Clear mem_path if we fall back to anonymous RAM allocation, Greg Kurz, 2018/04/19