linux-kernel - Re: [PATCH] mm: hugetlb: support get/set_policy for hugetlb_vm

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Date:   Tue, 18 Oct 2022 17:24:31 +0800
From:   黄杰 <huangjie.albert@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: hugetlb: support get/set_policy for hugetlb_vm_ops

Mike Kravetz <mike.kravetz@...cle.com> 于2022年10月18日周二 01:59写道：
>
> On 10/17/22 13:33, David Hildenbrand wrote:
> > On 17.10.22 11:48, 黄杰 wrote:
> > > David Hildenbrand <david@...hat.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@...edance.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.
>
> I absolutely agree with this cautionary note.
>
> hugetlb reservations guarantee that a sufficient number of huge pages exist.
> However, there is no guarantee that those pages are on any specific node
> associated with a numa policy.  Therefore, an 'on demand' allocation could
> fail resulting in SIGBUS being set to the faulting process.
> -

Yes, supporting on-demand requires adding a lot of other code to
support, I have thought about this, but there is currently no code
that is suitable for submitting to the community.


> Mike Kravetz