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From:   =?UTF-8?B?6buE5p2w?= <huangjie.albert@bytedance.com>
Date:   Mon, 17 Oct 2022 19:46:55 +0800
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Subject: Re: [External] Re: [PATCH] mm: hugetlb: support get/set_policy for hugetlb_vm_ops
To:     David Hildenbrand <david@redhat.com>
Cc:     songmuchun@bytedance.com, Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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David Hildenbrand <david@redhat.com> =E4=BA=8E2022=E5=B9=B410=E6=9C=8817=E6=
=97=A5=E5=91=A8=E4=B8=80 19:33=E5=86=99=E9=81=93=EF=BC=9A
>
> On 17.10.22 11:48, =E9=BB=84=E6=9D=B0 wrote:
> > David Hildenbrand <david@redhat.com> =E4=BA=8E2022=E5=B9=B410=E6=9C=881=
7=E6=97=A5=E5=91=A8=E4=B8=80 16:44=E5=86=99=E9=81=93=EF=BC=9A
> >>
> >> 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 qem=
u'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 trigger=
ed
> >>> 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 i=
s
> >> 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 bett=
er,
> > so the key point is to find a way support for shared policy=E3=80=82
>
> 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.

Thanks.

> --
> Thanks,
>
> David / dhildenb
>