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

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Date: Thu, 7 Mar 2024 12:10:43 +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@...ma-star.at, adobriyan@...il.com, 
	wangkefeng wang <wangkefeng.wang@...wei.com>, 
	ryan roberts <ryan.roberts@....com>, hughd@...gle.com, 
	peterx@...hat.com, avagin@...gle.com, 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>
> 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.

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.

> So best this can universally do is say "this PTE currently has write
> permissions".

Ok.

Thanks,
//richard