Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable

Richard Weinberger
2024-03-07T11:10:46Z

----- Ursprüngliche Mail -----
> Von: "David Hildenbrand" <david@redhat.com>
> But why is that required? What is the target use case? (I did not get
> the cover letter in my inbox)
>
> We're running slowly but steadily out of bits, so we better make wise
> decisions.
>
> Also, consider: Architectures where the dirty/access bit is not HW
> managed could indicate "writable" here although we *will* get a page
> fault to set the page dirty/accessed.

I'm currently investigating why a real-time application faces unexpected
page faults. Page faults are usually fatal for real-time work loads because
the latency constraints are no longer met.

So, I wrote a small tool to inspect the memory mappings of a process to find
areas which are not correctly pre-faulted. While doing so I noticed that
there is currently no way to detect CoW mappings.
Exposing the writable property of a PTE seemed like a good start to me.

> So best this can universally do is say "this PTE currently has write
> permissions".

Ok.

Thanks,
//richard