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

David Hildenbrand
2024-03-07T11:25:29Z

On 07.03.24 12:10, Richard Weinberger wrote:
> ----- 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.

Are you concerned about any type of page fault, or are things like a
simple remapping of the same page from "read-only to writable"
acceptable? ("very minor fault")

>
> 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.

Is it just about "detection" for debugging purposes or about "fixup" in
running applications?

If it's the latter, MADV_POPULATE_WRITE might do what you want (in
writable mappings).

--
Cheers,

David / dhildenb