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

[Mike Kravetz]

On 10/17/22 13:33, David Hildenbrand wrote:
> 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.

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.
-- 
Mike Kravetz