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SubjectHMM (Heterogeneous Memory Management) v7
So after PTO and before end of year frenzy here is an updated HMM patchset.
While not reusing Linus page table design, i use something that is, in my
view at least, close to it. Also i avoid pretending that this will be useful
to other and move it to hmm specific code. There is a longer justification
on why implementing a new page table code instead of using radix or other
existing kernel structure as part of commit message.

Everything else is pretty much the same, ie this patchset is just the ground
foundation on to which we want to build our features set. Main feature being
migrating memory to device memory. The very first version of this patchset
already show cased proof of concept of much of the features.

Below is previous patchset cover letter pretty much unchanged as background
and motivation for it did not.

What it is ?

In a nutshell HMM is a subsystem that provide an easy to use api to mirror a
process address on a device with minimal hardware requirement (mainly device
page fault and read only page mapping). This does not rely on ATS and PASID
PCIE extensions. It intends to supersede those extensions by allowing to move
system memory to device memory in a transparent fashion for core kernel mm
code (ie cpu page fault on page residing in device memory will trigger
migration back to system memory).

Why doing this ?

We want to be able to mirror a process address space so that compute api such
as OpenCL or other similar api can start using the exact same address space on
the GPU as on the CPU. This will greatly simplify usages of those api. Moreover
we believe that we will see more and more specialize unit functions that will
want to mirror process address using their own mmu.

The migration side is simply because GPU memory bandwidth is far beyond than
system memory bandwith and there is no sign that this gap is closing (quite the

Current status and future features :

None of this core code change in any major way core kernel mm code. This
is simple ground work with no impact on existing code path. Features that
will be implemented on top of this are :
1 - Tansparently handle page mapping on behalf of device driver (DMA).
2 - Improve DMA api to better match new usage pattern of HMM.
3 - Migration of anonymous memory to device memory.
4 - Locking memory to remote memory (CPU access trigger SIGBUS).
5 - Access exclusion btw CPU and device for atomic operations.
6 - Migration of file backed memory to device memory.

How future features will be implemented :
1 - Simply use existing DMA api to map page on behalf of a device.
2 - Introduce new DMA api to match new semantic of HMM. It is no longer page
we map but address range and managing which page is effectively backing
an address should be easy to update. I gave a presentation about that
during this LPC.
3 - Requires change to cpu page fault code path to handle migration back to
system memory on cpu access. An implementation of this was already sent
as part of v1. This will be low impact and only add a new special swap
type handling to existing fault code.
4 - Require a new syscall as i can not see which current syscall would be
appropriate for this. My first feeling was to use mbind as it has the
right semantic (binding a range of address to a device) but mbind is
too numa centric.

Second one was madvise, but semantic does not match, madvise does allow
kernel to ignore them while we do want to block cpu access for as long
as the range is bind to a device.

So i do not think any of existing syscall can be extended with new flags
but maybe i am wrong.
5 - Allowing to map a page as read only on the CPU while a device perform
some atomic operation on it (this is mainly to work around system bus
that do not support atomic memory access and sadly there is a large
base of hardware without that feature).

Easiest implementation would be using some page flags but there is none
left. So it must be some flags in vma to know if there is a need to query
HMM for write protection.

6 - This is the trickiest one to implement and while i showed a proof of
concept with v1, i am still have a lot of conflictual feeling about how
to achieve this.

As usual comments are more then welcome. Thanks in advance to anyone that
take a look at this code.

Previous patchset posting :
v2 (cover letter did not make it to ml)


To: "Andrew Morton" <>,
Cc: <>,
Cc: linux-mm <>,
Cc: <>,
Cc: "Linus Torvalds" <>,
Cc: "Mel Gorman" <>,
Cc: "H. Peter Anvin" <>,
Cc: "Peter Zijlstra" <>,
Cc: "Linda Wang" <>,
Cc: "Kevin E Martin" <>,
Cc: "Jerome Glisse" <>,
Cc: "Andrea Arcangeli" <>,
Cc: "Johannes Weiner" <>,
Cc: "Larry Woodman" <>,
Cc: "Rik van Riel" <>,
Cc: "Dave Airlie" <>,
Cc: "Jeff Law" <>,
Cc: "Brendan Conoboy" <>,
Cc: "Joe Donohue" <>,
Cc: "Duncan Poole" <>,
Cc: "Sherry Cheung" <>,
Cc: "Subhash Gutti" <>,
Cc: "John Hubbard" <>,
Cc: "Mark Hairgrove" <>,
Cc: "Lucien Dunning" <>,
Cc: "Cameron Buschardt" <>,
Cc: "Arvind Gopalakrishnan" <>,
Cc: "Haggai Eran" <>,
Cc: "Or Gerlitz" <>,
Cc: "Sagi Grimberg" <>
Cc: "Shachar Raindel" <>,
Cc: "Liran Liss" <>,
Cc: "Roland Dreier" <>,
Cc: "Sander, Ben" <>,
Cc: "Stoner, Greg" <>,
Cc: "Bridgman, John" <>,
Cc: "Mantor, Michael" <>,
Cc: "Blinzer, Paul" <>,
Cc: "Morichetti, Laurent" <>,
Cc: "Deucher, Alexander" <>,
Cc: "Gabbay, Oded" <>,

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