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

[Richard Weinberger]

----- Ursprüngliche Mail -----
> Von: "David Hildenbrand" <david@xxxxxxxxxx>
>> 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")

Any page fault has to be avoided.
To give you more background, the real time application runs on Xenomai,
a real time extension for Linux.
Xenomai applies already many tweaks to the kernel to trigger pre-faulting of
memory areas. But sometimes the application does not use the Xenomai API
correctly or there is an bug in Xenomai it self.
Currently I'm suspecting the latter.
 
>> 
>> 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?

It's only about debugging. If an application fails a test I want to have
a tool which tells me what memory mappings are wonky or could cause a fault
at runtime.

I fully understand that my use case is a corner case and anything but mainline.
While developing my debug tool I thought that improving the pagemap interface
might help others too.

Thanks,
//richard