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Date: Wed, 31 Oct 2018 15:57:21 -0700
From: Andy Lutomirski <>
To: Peter Zijlstra <>
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 <>
Subject: Re: [PATCH 10/17] prmem: documentation

> On Oct 31, 2018, at 2:00 PM, Peter Zijlstra <> wrote:
>> On Wed, Oct 31, 2018 at 01:36:48PM -0700, Andy Lutomirski wrote:
>>>> On Oct 31, 2018, at 3:02 AM, Peter Zijlstra <> 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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