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([2601:646:c200:7429:3d1b:ef7:2c4c:5b1]) by smtp.gmail.com with ESMTPSA id m19-v6sm27298910pgv.93.2018.10.31.15.57.22 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 31 Oct 2018 15:57:22 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (1.0) Subject: Re: [PATCH 10/17] prmem: documentation From: Andy Lutomirski X-Mailer: iPhone Mail (16A404) In-Reply-To: <20181031210032.GA3159@hirez.programming.kicks-ass.net> Date: Wed, 31 Oct 2018 15:57:21 -0700 Cc: Matthew Wilcox , Igor Stoppa , Tycho Andersen , Kees Cook , Mimi Zohar , Dave Chinner , James Morris , Michal Hocko , Kernel Hardening , linux-integrity , LSM List , Igor Stoppa , Dave Hansen , Jonathan Corbet , Laura Abbott , Randy Dunlap , Mike Rapoport , "open list:DOCUMENTATION" , LKML , Thomas Gleixner Content-Transfer-Encoding: quoted-printable Message-Id: <2E29C35B-3F98-473D-A874-0D9B9B15B399@amacapital.net> References: <0A7AFB50-9ADE-4E12-B541-EC7839223B65@amacapital.net> <20181030175814.GB10491@bombadil.infradead.org> <20181030182841.GE7343@cisco> <20181030192021.GC10491@bombadil.infradead.org> <9edbdf8b-b5fb-5a82-43b4-b639f5ec8484@gmail.com> <20181030213557.GE10491@bombadil.infradead.org> <20181031100237.GN744@hirez.programming.kicks-ass.net> <659CFC78-22BF-492B-B2E4-B8E89AA08446@amacapital.net> <20181031210032.GA3159@hirez.programming.kicks-ass.net> To: Peter Zijlstra Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Oct 31, 2018, at 2:00 PM, Peter Zijlstra 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 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?=