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

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Date: Thu, 7 Mar 2024 12:20:17 +0100
From: David Hildenbrand <david@...hat.com>
To: Richard Weinberger <richard@....at>
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

On 07.03.24 12:10, Richard Weinberger wrote:
> ----- 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.

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