Date:   Thu, 20 Jul 2017 13:18:50 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Yisheng Xie <xieyisheng1@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Nellans <dnellans@...dia.com>
Subject: Re: [PATCH 00/15] HMM (Heterogeneous Memory Management) v24

On Wed, Jul 19, 2017 at 07:48:08PM +0800, Yisheng Xie wrote:
> Hi Jérôme
> 
> On 2017/6/29 2:00, Jérôme Glisse 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-v24
> > 
> > Change since v23 is code comment fixes, simplify kernel configuration and
> > improve allocation of new page on migration do device memory (last patch
> > in this patchset).
> > 
> > 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
> > that rely on pining pages use by a device, HMM relies on mmu_notifier to
> > propagate CPU page table update to device page table.
> > 
> > Duplicating CPU page table is only one aspect necessary for efficiently
> > using device like GPU. GPU local memory have bandwidth in the TeraBytes/
> > second range but they are connected to main memory through a system bus
> > like PCIE that is limited to 32GigaBytes/second (PCIE 4.0 16x). Thus it
> > is necessary to allow migration of process memory from main system memory
> > to device memory. Issue is that on platform that only have PCIE the device
> > memory is not accessible by the CPU with the same properties as main
> > memory (cache coherency, atomic operations, ...).
> > 
> > To allow migration from main memory to device memory HMM provides a set
> > of helper to hotplug device memory as a new type of ZONE_DEVICE memory
> > which is un-addressable by CPU but still has struct page representing it.
> > This allow most of the core kernel logic that deals with a process memory
> > to stay oblivious of the peculiarity of device memory.
> > 
> > When page backing an address of a process is migrated to device memory
> > the CPU page table entry is set to a new specific swap entry. CPU access
> > to such address triggers a migration back to system memory, just like if
> > the page was swap on disk. 
> > [...]
> > To allow efficient migration between device memory and main memory a new
> > migrate_vma() helpers is added with this patchset. It allows to leverage
> > device DMA engine to perform the copy operation.
> > 
> 
> Is this means that when CPU access an address of a process is migrated to device
> memory, it should call migrate_vma() to migrate a range of address back to CPU ?
> If it is so, I think it should somewhere call this function in this patchset,
> however, I do not find anywhere in this patchset call this function.
> 
> Or am I miss anything?

There is a callback in struct dev_pagemap page_fault. Device driver will
set that callback to a device driver function that itself might call
migrate_vma(). It might call a different helper thought.

For instance GPU driver commonly use memory oversubscription, ie they
evict device memory to system page to make room for other stuff. If a
page fault happen while there is already a system page for that memory
than the device driver might only need to hand over that page and no
need to migrate anything.

That is why you do not see migrate_vma() call in this patchset. Calls
to that function will be inside the individual device driver.

Cheers,
Jérôme

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