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Message-Id: <2E29C35B-3F98-473D-A874-0D9B9B15B399@amacapital.net>
Date: Wed, 31 Oct 2018 15:57:21 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Peter Zijlstra <peterz@...radead.org>
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 Oct 31, 2018, at 2:00 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
>> 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.
Sounds nuts. Doing a rare-write is many hundreds of cycles at best. Using that for a reference count sounds wacky.
Can we see a *real* use case before we over complicate the API?
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