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

[David Hildenbrand (Red Hat)]

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