linux-kernel - Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18

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Date:   Thu, 16 Mar 2017 13:43:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        John Hubbard <jhubbard@...dia.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        David Nellans <dnellans@...dia.com>
Subject: Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18

On Thu, 16 Mar 2017 12:05:19 -0400 J__r__me Glisse <jglisse@...hat.com> wrote:

> Cliff note:

"Cliff's notes" isn't appropriate for a large feature such as this. 
Where's the long-form description?  One which permits readers to fully
understand the requirements, design, alternative designs, the
implementation, the interface(s), etc?

Have you ever spoken about HMM at a conference?  If so, the supporting
presentation documents might help here.  That's the level of detail
which should be presented here.

> HMM offers 2 things (each standing on its own). First
> it allows to use device memory transparently inside any process
> without any modifications to process program code.

Well.  What is "device memory"?  That's very vague.  What are the
characteristics of this memory?  Why is it a requirement that
userspace code be unaltered?  What are the security implications - does
the process need particular permissions to access this memory?  What is
the proposed interface to set up this access?

> Second it allows to mirror process address space on a device.

Why?  Why is this a requirement, how will it be used, what are the
use cases, etc?

I spent a bit of time trying to locate a decent writeup of this feature
but wasn't able to locate one.  I'm not seeing a Documentation/ update
in this patchset.  Perhaps if you were to sit down and write a detailed
Documentation/vm/hmm.txt then that would be a good starting point.

This stuff is important - it's not really feasible to perform a decent
review of this proposal unless the reviewer has access to this
high-level conceptual stuff.

So I'll take a look at merging this code as-is for testing purposes but
I won't be attempting to review it at this stage.