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

zhangqilong
2025-12-16T07:42:53Z

> 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?

It's really a good idea. Maybe could setting a xx flag to VMA, but it need hold write
lock and touch VMA. let me have a think how to implement per-VMA or per-file policy
or other better hints.

I have a thought that, how about adding a new flag(in ptrace, uprobes...) and passing
to faultin_page() to mark the exec PMD COW policy?

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

Year, good catch here, it's really need be seriously considered.

>
> --
> Cheers
>
> David