Date:   Fri, 23 Jun 2017 11:28:50 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Bob Liu <lliubbo@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [HMM 00/15] HMM (Heterogeneous Memory Management) v23

On Fri, Jun 23, 2017 at 11:00:37PM +0800, Bob Liu wrote:
> Hi,
> 
> On Thu, May 25, 2017 at 1:20 AM, Jérôme Glisse <jglisse@...hat.com> wrote:
> > Patchset is on top of git://git.cmpxchg.org/linux-mmotm.git so i
> > test same kernel as kbuild system, git branch:
> >
> > https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-v23
> >
> > Change since v22 is use of static key for special ZONE_DEVICE case in
> > put_page() and build fix for architecture with no mmu.
> >
> > Everything else is the same. Below is the long description of what HMM
> > is about and why. At the end of this email i describe briefly each patch
> > and suggest reviewers for each of them.
> >
> >
> > Heterogeneous Memory Management (HMM) (description and justification)
> >
> > Today device driver expose dedicated memory allocation API through their
> > device file, often relying on a combination of IOCTL and mmap calls. The
> > device can only access and use memory allocated through this API. This
> > effectively split the program address space into object allocated for the
> > device and useable by the device and other regular memory (malloc, mmap
> > of a file, share memory, â) only accessible by CPU (or in a very limited
> > way by a device by pinning memory).
> >
> > Allowing different isolated component of a program to use a device thus
> > require duplication of the input data structure using device memory
> > allocator. This is reasonable for simple data structure (array, grid,
> > image, â) but this get extremely complex with advance data structure
> > (list, tree, graph, â) that rely on a web of memory pointers. This is
> > becoming a serious limitation on the kind of work load that can be
> > offloaded to device like GPU.
> >
> > New industry standard like C++, OpenCL or CUDA are pushing to remove this
> > barrier. This require a shared address space between GPU device and CPU so
> > that GPU can access any memory of a process (while still obeying memory
> > protection like read only). This kind of feature is also appearing in
> > various other operating systems.
> >
> > HMM is a set of helpers to facilitate several aspects of address space
> > sharing and device memory management. Unlike existing sharing mechanism
> 
> It looks like the address space sharing and device memory management
> are two different things. They don't depend on each other and HMM has
> helpers for both.
> 
> Is it possible to separate these two things into two patchsets?
> Which will make it's more easy to review and also follow the "Do one
> thing, and do it well" philosophy.
> 

They are already seperate. Patch 3-5 are for address space mirroring.
Patch 6-10 for device memory using struct page and ZONE_DEVICE. Finaly
patch 11-15 for adding new page migration helper capable of using
device DMA engine to perform memory copy operation.

Patch 1 is just common documentation and patch 2 is common helpers and
definitions.

Also they are separate at kernel configuration level. So for all intents
and purposes this is already 2 separate things, just in one posting
because first user will use both. You can use one without the other and
it will work properly.

Cheers,
Jérôme

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