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Message-ID: <20181031210032.GA3159@hirez.programming.kicks-ass.net>
Date: Wed, 31 Oct 2018 22:00:32 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: Matthew Wilcox <willy@...radead.org>,
Igor Stoppa <igor.stoppa@...il.com>,
Tycho Andersen <tycho@...ho.ws>, Kees Cook <keescook@...omium.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>,
Igor Stoppa <igor.stoppa@...wei.com>,
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 Wed, Oct 31, 2018 at 01:36:48PM -0700, Andy Lutomirski wrote:
>
> > On Oct 31, 2018, at 3:02 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> >> On Tue, Oct 30, 2018 at 09:41:13PM -0700, Andy Lutomirski wrote:
> >> To clarify some of this thread, I think that the fact that rare_write
> >> uses an mm_struct and alias mappings under the hood should be
> >> completely invisible to users of the API. No one should ever be
> >> handed a writable pointer to rare_write memory (except perhaps during
> >> bootup or when initializing a large complex data structure that will
> >> be rare_write but isn't yet, e.g. the policy db).
> >
> > Being able to use pointers would make it far easier to do atomics and
> > other things though.
>
> This stuff is called *rare* write for a reason. Do we really want to
> allow atomics beyond just store-release? Taking a big lock and then
> writing in the right order should cover everything, no?
Ah, so no. That naming is very misleading.
We modify page-tables a _lot_. The point is that only a few sanctioned
sites are allowed writing to it, not everybody.
I _think_ the use-case for atomics is updating the reference counts of
objects that are in this write-rare domain. But I'm not entirely clear
on that myself either. I just really want to avoid duplicating that
stuff.
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