On 07.03.24 12:10, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
>> Von: "David Hildenbrand" <[email protected]>
>> 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
----- Ursprüngliche Mail -----
> Von: "David Hildenbrand" <[email protected]>
>> 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