Re: [PATCH] mm: hugetlb: support get/set_policy for hugetlb_vm_ops

[David Hildenbrand <david@redhat.com>]

On 18.10.22 11:27, 黄杰 wrote:
> David Hildenbrand <david@redhat.com> 于2022年10月17日周一 20:00写道：
>>
>> On 17.10.22 13:46, 黄杰 wrote:
>>> David Hildenbrand <david@redhat.com> 于2022年10月17日周一 19:33写道：
>>>>
>>>> On 17.10.22 11:48, 黄杰 wrote:
>>>>> David Hildenbrand <david@redhat.com> 于2022年10月17日周一 16:44写道：
>>>>>>
>>>>>> On 12.10.22 10:15, Albert Huang wrote:
>>>>>>> From: "huangjie.albert" <huangjie.albert@bytedance.com>
>>>>>>>
>>>>>>> implement these two functions so that we can set the mempolicy to
>>>>>>> the inode of the hugetlb file. This ensures that the mempolicy of
>>>>>>> all processes sharing this huge page file is consistent.
>>>>>>>
>>>>>>> In some scenarios where huge pages are shared:
>>>>>>> if we need to limit the memory usage of vm within node0, so I set qemu's
>>>>>>> mempilciy bind to node0, but if there is a process (such as virtiofsd)
>>>>>>> shared memory with the vm, in this case. If the page fault is triggered
>>>>>>> by virtiofsd, the allocated memory may go to node1 which  depends on
>>>>>>> virtiofsd.
>>>>>>>
>>>>>>
>>>>>> Any VM that uses hugetlb should be preallocating memory. For example,
>>>>>> this is the expected default under QEMU when using huge pages.
>>>>>>
>>>>>> Once preallocation does the right thing regarding NUMA policy, there is
>>>>>> no need to worry about it in other sub-processes.
>>>>>>
>>>>>
>>>>> Hi, David
>>>>> thanks for your reminder
>>>>>
>>>>> Yes, you are absolutely right, However, the pre-allocation mechanism
>>>>> does solve this problem.
>>>>> However, some scenarios do not like to use the pre-allocation mechanism, such as
>>>>> scenarios that are sensitive to virtual machine startup time, or
>>>>> scenarios that require
>>>>> high memory utilization. The on-demand allocation mechanism may be better,
>>>>> so the key point is to find a way support for shared policy。
>>>>
>>>> Using hugetlb -- with a fixed pool size -- without preallocation is like
>>>> playing with fire. Hugetlb reservation makes one believe that on-demand
>>>> allocation is going to work, but there are various scenarios where that
>>>> can go seriously wrong, and you can run out of huge pages.
>>>>
>>>> If you're using hugetlb as memory backend for a VM without
>>>> preallocation, you really have to be very careful. I can only advise
>>>> against doing that.
>>>>
>>>>
>>>> Also: why does another process read/write *first* to a guest physical
>>>> memory location before the OS running inside the VM even initialized
>>>> that memory? That sounds very wrong. What am I missing?
>>>>
>>>
>>> for example : virtio ring buffer.
>>> For the avial descriptor, the guest kernel only gives an address to
>>> the backend,
>>> and does not actually access the memory.
>>
>> Okay, thanks. So we're essentially providing uninitialized memory to a
>> device? Hm, that implies that the device might have access to memory
>> that was previously used by someone else ... not sure how to feel about
>> that, but maybe this is just the way of doing things.
>>
>> The "easy" user-space fix would be to simply similarly mbind() in  the
>> other processes where we mmap(). Has that option been explored?
> 
> This can also solve the problem temporarily, but we need to change all
> processes that share memory with it, so it can't be done once and for
> all

How many process that really care are we walking about? Usually, 
extending the vhost-user protocol and relevant libraries should be good 
enough.

-- 
Thanks,

David / dhildenb