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

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Date: Thu, 7 Mar 2024 15:42:49 +0100 (CET)
From: Richard Weinberger <richard@....at>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm <linux-mm@...ck.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, 
	linux-kernel <linux-kernel@...r.kernel.org>, 
	Linux Doc Mailing List <linux-doc@...r.kernel.org>, 
	upstream+pagemap <upstream+pagemap@...ma-star.at>, 
	adobriyan <adobriyan@...il.com>, 
	wangkefeng wang <wangkefeng.wang@...wei.com>, 
	ryan roberts <ryan.roberts@....com>, hughd <hughd@...gle.com>, 
	peterx <peterx@...hat.com>, avagin <avagin@...gle.com>, 
	lstoakes <lstoakes@...il.com>, vbabka <vbabka@...e.cz>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	usama anjum <usama.anjum@...labora.com>, 
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is
 writable

----- Ursprüngliche Mail -----
> Von: "David Hildenbrand" <david@...hat.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