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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