Re: [PATCH next 0/2] THP COW support for private executable file mmap

David Hildenbrand (Red Hat)
2025-12-16T02:43:12Z

On 12/16/25 03:24, zhangqilong wrote:
> > On 12/15/25 15:00, Matthew Wilcox wrote:
>>> On Mon, Dec 15, 2025 at 08:34:05PM +0800, Zhang Qilong wrote:
>>>> This patch series implementate THP COW for private executable file
>>>> mmap. It's major designed to increase the iTLB cache hit rate for hot
>>>> patching application, and we add a new sysfs knob to disable or
>>>> enable it.
>>>
>>> You're going to have to provide data to get this patch in. We've
>>> deliberately not done this in the past due to memory consumption
>> overhead.
>>> So you need to prove that's now the wrong decision to make.
>>>
>>> Microbenchmarks would be a bare minimum, but what are really needed
>>> are numbers from actual workloads.
>>
>> In addition, the sysfs toggle is rather horrible. It's rather clear that this is not a
>> system-wide setting to be made, as you likely only want that behavior (if at
>> all ...) for a handful of special processes I assume?
>
> Year, it's not a system-wide setting. We consider enabling this option only when
> applying hot patches to special processes. If the sysfs toggle is unavailable, we will
> evaluate the overall memory impact on the system after removing it. Thanks very
> much for your suggestion.

I don't think we want this as any kind of default behavior. But the
system toggle is really also not what we want. Could we use some per-VMA
or per-file hints to affect the policy?

Note that your proposal will likely interact in bad ways with uprobes,
after removing uprobes again.

--
Cheers

David