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

[David Hildenbrand]

On 18.10.22 11:27, 黄杰 wrote:
> David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > > > > >
> > > > > > > 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