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Subject: Re: [PATCH 10/17] prmem: documentation
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181031210032.GA3159@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Oct 2018 15:57:21 -0700
Cc:     Matthew Wilcox <willy@infradead.org>,
        Igor Stoppa <igor.stoppa@gmail.com>,
        Tycho Andersen <tycho@tycho.ws>,
        Kees Cook <keescook@chromium.org>,
        Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Dave Chinner <david@fromorbit.com>,
        James Morris <jmorris@namei.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-integrity <linux-integrity@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Igor Stoppa <igor.stoppa@huawei.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Laura Abbott <labbott@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Oct 31, 2018, at 2:00 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
>> On Wed, Oct 31, 2018 at 01:36:48PM -0700, Andy Lutomirski wrote:
>>=20
>>>> On Oct 31, 2018, at 3:02 AM, Peter Zijlstra <peterz@infradead.org> wrot=
e:
>>>>=20
>>>> 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).
>>>=20
>>> Being able to use pointers would make it far easier to do atomics and
>>> other things though.
>>=20
>> 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?
>=20
> Ah, so no. That naming is very misleading.
>=20
> We modify page-tables a _lot_. The point is that only a few sanctioned
> sites are allowed writing to it, not everybody.
>=20
> 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 th=
at for a reference count sounds wacky.

Can we see a *real* use case before we over complicate the API?=