linux-kernel - Re: [PATCH 10/17] prmem: documentation

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Date:   Thu, 22 Nov 2018 12:04:16 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
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.