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Date: Thu, 7 Mar 2024 15:42:49 +0100 (CET)
From: Richard Weinberger <richard@nod.at>
To: David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is
 writable
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Thread-Topic: proc: pagemap: Expose whether a PTE is writable
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----- Urspr=C3=BCngliche Mail -----
> Von: "David Hildenbrand" <david@redhat.com>
>> One destructive way to find out in a writable mapping if the page would
>> actually get remapped:
>>=20
>> 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
>>=20
>> 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 su=
ch.
=20
>> 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 t=
hread
had the CPU.
My idea was having a tool to check before the applications enters the criti=
cal phase.

>>> I fully understand that my use case is a corner case and anything but m=
ainline.
>>> While developing my debug tool I thought that improving the pagemap int=
erface
>>> might help others too.
>>=20
>> 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. :)
=20
>=20
> [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() implementat=
ions.
That's a tradeoff I can live with.

Thanks,
//richard