Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 1/13/26 13:41, Li Zhe wrote:
> On Tue, 13 Jan 2026 11:15:29 +0100, david@kernel.org wrote:
> 
>> On 1/13/26 07:37, Li Zhe wrote:
>>> On Mon, 12 Jan 2026 20:52:12 +0100, david@kernel.org wrote:
>>>
>>>>> As for concern (4), I believe it is orthogonal to this patchset, and
>>>>> the cover letter already contains a performance comparison that
>>>>> demonstrates the additional benefit.
>>>>>
>>>>>> I did see some comments in [1] about QEMU supporting user-mode
>>>>>> parallel zero-page operations; I'm just not sure what the current
>>>>>> state of that support looks like, or what the corresponding benchmark
>>>>>> numbers are.
>>>>>
>>>>> As noted above, QEMU already employs a parallel page-touch mechanism,
>>>>> yet the elapsed time remains noticeable. I am not deeply familiar with
>>>>> QEMU; please correct me if I am mistaken.
>>>>
>>>> I implemented some part of the parallel preallocation support in QEMU.
>>>>
>>>> With QEMU, you can specify the number of threads and even specify the
>>>> NUMA-placement of these threads. So you can pretty much fine-tune that
>>>> for an environment.
>>>>
>>>> You still pre-zero all hugetlb pages at VM startup time, just in
>>>> parallel though. So you pay some price at APP startup time.
>>>
>>> Hi David,
>>>
>>> Thank you for the comprehensive explanation.
>>>
>>> You are absolutely correct: QEMU's parallel preallocation is performed
>>> only during VM start-up. We submitted this patch series mainly
>>> because we observed that, even with the existing parallel mechanism,
>>> launching large-size VMs still incurs prohibitive delays. (Bringing up
>>> a 2 TB VM still requires more than 40 seconds for zeroing)
>>>
>>>> If you know that you will run such a VM (or something else) later, you
>>>> could pre-zero the memory from user space by using a hugetlb-backed file
>>>> and supplying that to QEMU as memory backend for the VM. Then, you can
>>>> start your VM without any pre-zeroing.
>>>>
>>>> I guess that approach should work universally. Of course, there are
>>>> limitations, as you would have to know how much memory an app needs, and
>>>> have a way to supply that memory in form of a file to that app.
>>>
>>> Regarding user-space pre-zeroing, I agree that it is feasible once the
>>> VM's memory footprint is known. We evaluated this approach internally;
>>> however, in production environments, it is almost impossible to predict
>>> the exact amount of memory a VM will require.
>>
>> Of course, you could preallocate to the expected maximum and then
>> truncate the file to the size you need :)
> 
> The solution you described seems similar to delegating hugepage
> management to a userspace daemon. I haven't explored this approach
> before, but it appears quite complex. Beyond ensuring secure memory
> isolation between VMs, we would also need to handle scenarios where
> the management daemon or the QEMU process crashes, which implies
> implementing robust recovery and memory reclamation mechanisms. 

Yes, but I don't think that's particularly complicated. You have to 
remove the backing file, yes.

> Do
> you happen to have any documentation or references regarding
> userspace hugepage management that I could look into? 

Not really any documentation. I pretty much only know how QEMU+libvirt 
ends up using it :)

> Compared to
> the userspace approach, I wonder if implementing hugepage
> pre-zeroing directly within the kernel would be a simpler and more
> direct way to accelerate VM creation.

I mean, yes. I don't particularly enjoy user-space having to poll for 
pre-zeroing of pages ... it feels like an odd interface for something 
that is supposed to be simple.

I do understand the reasoning that "zeroing must be charged to 
somebody", and that using a kthread is a bit suboptimal as well.


Here is a thought: with "init_on_free", we charge zeroing of pages to 
whoever frees a page.

Can't we have a hugetlb mode where we zero hugetlb folios as they are 
getting freed back to the hugetlb allcoator? IOW, we charge it to 
whoever puts the last reference.

just a thought, maybe it was discussed before ...

-- 
Cheers

David