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Message-ID: <20181122200416.GS3065@bombadil.infradead.org>
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.
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