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Date: Thu, 22 Nov 2018 12:04:16 -0800
From: Matthew Wilcox <>
To: Igor Stoppa <>
Cc: Andy Lutomirski <>,
	Andy Lutomirski <>,
	Igor Stoppa <>,
	Nadav Amit <>,
	Kees Cook <>,
	Peter Zijlstra <>,
	Mimi Zohar <>,
	Dave Chinner <>,
	James Morris <>, Michal Hocko <>,
	Kernel Hardening <>,
	linux-integrity <>,
	LSM List <>,
	Dave Hansen <>,
	Jonathan Corbet <>, Laura Abbott <>,
	Randy Dunlap <>,
	Mike Rapoport <>,
	"open list:DOCUMENTATION" <>,
	LKML <>,
	Thomas Gleixner <>
Subject: Re: [PATCH 10/17] prmem: documentation

On Thu, Nov 22, 2018 at 09:27:02PM +0200, Igor Stoppa wrote:
> I have studied the code involved with Nadav's patchset.
> I am perplexed about these sentences you wrote.
> More to the point (to the best of my understanding):
> poking_init()
> -------------
>   1. it gets one random poking address and ensures to have at least 2
>      consecutive PTEs from the same PMD
>   2. it then proceeds to map/unmap an address from the first of the 2
>      consecutive PTEs, so that, later on, there will be no need to
>      allocate pages, which might fail, if poking from atomic context.
>   3. at this point, the page tables are populated, for the address that
>      was obtained at point 1, and this is ok, because the address is fixed
> write_rare
> ----------
>   4. it can happen on any available core / thread at any time, therefore
>      each of them needs a different address

No?  Each CPU has its own CR3 (eg each CPU might be running a different
user task).  If you have _one_ address for each allocation, it may or
may not be mapped on other CPUs at the same time -- you simply don't care.

The writable address can even be a simple formula to calculate from
the read-only address, you don't have to allocate an address in the
writable mapping space.

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