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

Richard Weinberger
2024-03-07T14:42:52Z

----- Ursprüngliche Mail -----
> Von: "David Hildenbrand" <david@redhat.com>
>> One destructive way to find out in a writable mapping if the page would
>> actually get remapped:
>>
>> a) Read the PFN of a virtual address using pagemap
>> b) Write to the virtual address using /proc/pid/mem
>> c) Read the PFN of a virtual address using pagemap to see if it changed
>>
>> If the application can be paused, you could read+write a single byte,
>> turning it non-destructive.

I'm not so sure whether this works well if a mapping is device memory or such.

>> But that would still "hide" the remap-writable-type faults.

Xenomai will tell me anyway when there was a page fault while a real time thread
had the CPU.
My idea was having a tool to check before the applications enters the critical phase.

>>> 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.
>>
>> I'm fine with this (can be a helpful debugging tool for some other cases
>> as well, and IIRC we don't have another interface to introspect this),
>> as long as we properly document the corner case that there could still
>> be writefaults on some architectures when the page would not be
>> accessed/dirty yet.

Cool. :)

>
> [and I just recall, there are some other corner cases. For example,
> pages in a shadow stack can be pte_write(), but they can only be written
> by HW indirectly when modifying the stack, and ordinary write access
> would still fault]

Yeah, I noticed this while browsing through various pte_write() implementations.
That's a tradeoff I can live with.

Thanks,
//richard