[RFC 0/8] Define coherent device memory node

[Anshuman Khandual]

	There are certain devices like accelerators, GPU cards, network
cards, FPGA cards, PLD cards etc which might contain on board memory. This
on board memory can be coherent along with system RAM and may be accessible
from either the CPU or from the device. The coherency is usually achieved
through synchronizing the cache accesses from either side. This makes the
device memory appear in the same address space as that of the system RAM.
The on board device memory and system RAM are coherent but have differences
in their properties as explained and elaborated below. Following diagram
explains how the coherent device memory appears in the memory address
space.

                +-----------------+         +-----------------+
                |                 |         |                 |
                |       CPU       |         |     DEVICE      |
                |                 |         |                 |
                +-----------------+         +-----------------+
                         |                           |
                         |   Shared Address Space    |
 +---------------------------------------------------------------------+
 |                                             |                       |
 |                                             |                       |
 |                 System RAM                  |     Coherent Memory   |
 |                                             |                       |
 |                                             |                       |
 +---------------------------------------------------------------------+

	User space applications might be interested in using the coherent
device memory either explicitly or implicitly along with the system RAM
utilizing the basic semantics for memory allocation, access and release.
Basically the user applications should be able to allocate memory any where
(system RAM or coherent memory) and then get it accessed either from the
CPU or from the coherent device for various computation or data
transformation purpose. User space really should not be concerned about
memory placement and their subsequent allocations when the memory really
faults because of the access.

	To achieve seamless integration  between system RAM and coherent
device memory it must be able to utilize core memory kernel features like
anon mapping, file mapping, page cache, driver managed pages, HW poisoning,
migrations, reclaim, compaction, etc. Making the coherent device memory
appear as a distinct memory only NUMA node which will be initialized as any
other node with memory can create this integration with currently available
system RAM memory. Also at the same time there should be a differentiating
mark which indicates that this node is a coherent device memory node not
any other memory only system RAM node.
 
	Coherent device memory invariably isn't available until the driver
for the device has been initialized. It is desirable but not required for
the device to support memory offlining for the purposes such as power
management, link management and hardware errors. Kernel allocation should
not come here as it cannot be moved out. Hence coherent device memory
should go inside ZONE_MOVABLE zone instead. This guarantees that kernel
allocations will never be satisfied from this memory and any process having
un-movable pages on this coherent device memory (likely achieved through
pinning later on after initial allocation) can be killed to free up memory
from page table and eventually hot plugging the node out.

	After similar representation as a NUMA node, the coherent memory
might still need some special consideration while being inside the kernel.
There can be a variety of coherent device memory nodes with different
expectations and special considerations from the core kernel. This RFC
discusses only one such scenario where the coherent device memory requires
just isolation.

	Now let us consider in detail the case of a coherent device memory
node which requires isolation. This kind of coherent device memory is on
board an external device attached to the system through a link where there
is a chance of link errors plugging out the entire memory node with it.
More over the memory might also have higher chances of ECC errors as
compared to the system RAM. These are just some possibilities. But the fact
remains that the coherent device memory can have some other different
properties which might not be desirable for some user space applications.
An application should not be exposed to related risks of a device if its
not taking advantage of special features of that device and it's memory.

	Because of the reasons explained above allocations into isolation
based coherent device memory node should further be regulated apart from
earlier requirement of kernel allocations not coming there. User space
allocations should not come here implicitly without the user application
explicitly knowing about it. This summarizes isolation requirement of
certain kind of a coherent device memory node as an example.

	Some coherent memory devices may not require isolation altogether.
Then there might be other coherent memory devices which require some other
special treatment after being part of core memory representation in kernel.
Though the framework suggested by this RFC has made provisions for them, it
has not considered any other kind of requirement other than isolation for
now.

	Though this RFC series currently attempts to implement one such
isolation seeking coherent device memory example, this framework can be
extended to accommodate any present or future coherent memory devices which
will fit the requirement as explained before even with new requirements
other than isolation. In case of isolation seeking coherent device memory
node, there will be other core VM code paths which need to be taken care
before it can be completely isolated as required.

	Core kernel memory features like reclamation, evictions etc. might
need to be restricted or modified on the coherent device memory node as
they can be performance limiting. The RFC does not propose anything on this
yet but it can be looked into later on. For now it just disables Auto NUMA
for any VMA which has coherent device memory.

	Seamless integration of coherent device memory with system memory
will enable various other features, some of which can be listed as follows.

	a. Seamless migrations between system RAM and the coherent memory
	b. Will have asynchronous and high throughput migrations
	c. Be able to allocate huge order pages from these memory regions
	d. Restrict allocations to a large extent to the tasks using the
	   device for workload acceleration

	Before concluding, will look into the reasons why the existing
solutions don't work. There are two basic requirements which have to be
satisfies before the coherent device memory can be integrated with core
kernel seamlessly.

	a. PFN must have struct page
	b. Struct page must able to be inside standard LRU lists

	The above two basic requirements discard the existing method of
device memory representation approaches like these which then requires the
need of creating a new framework.

(1) Traditional ioremap

	a. Memory is mapped into kernel (linear and virtual) and user space
	b. These PFNs do not have struct pages associated with it
	c. These special PFNs are marked with special flags inside the PTE
	d. Cannot participate in core VM functions much because of this
	e. Cannot do easy user space migrations

(2) Zone ZONE_DEVICE

	a. Memory is mapped into kernel and user space
	b. PFNs do have struct pages associated with it
	c. These struct pages are allocated inside it's own memory range
	d. Unfortunately the struct page's union containing LRU has been
	   used for struct dev_pagemap pointer
	e. Hence it cannot be part of any LRU (like Page cache)
	f. Hence file cached mapping cannot reside on these PFNs
	g. Cannot do easy migrations

	I had also explored non LRU representation of this coherent device
memory where the integration with system RAM in the core VM is limited only
to the following functions. Not being inside LRU is definitely going to
reduce the scope of tight integration with system RAM.

(1) Migration support between system RAM and coherent memory
(2) Migration support between various coherent memory nodes
(3) Isolation of the coherent memory
(4) Mapping the coherent memory into user space through driver's
    struct vm_operations
(5) HW poisoning of the coherent memory

	Allocating the entire memory of the coherent device node right
after hot plug into ZONE_MOVABLE (where the memory is already inside the
buddy system) will still expose a time window where other user space
allocations can come into the coherent device memory node and prevent the
intended isolation. So traditional hot plug is not the solution. Hence
started looking into CMA based non LRU solution but then hit the following
roadblocks.

(1) CMA does not support hot plugging of new memory node
	a. CMA area needs to be marked during boot before buddy is
	   initialized
	b. cma_alloc()/cma_release() can happen on the marked area
	c. Should be able to mark the CMA areas just after memory hot plug
	d. cma_alloc()/cma_release() can happen later after the hot plug
	e. This is not currently supported right now

(2) Mapped non LRU migration of pages
	a. Recent work from Michan Kim makes non LRU page migratable
	b. But it still does not support migration of mapped non LRU pages
	c. With non LRU CMA reserved, again there are some additional
	   challenges

	With hot pluggable CMA and non LRU mapped migration support there
may be an alternate approach to represent coherent device memory. Please
do review this RFC proposal and let me know your comments or suggestions.
Thank you.

Anshuman Khandual (8):
  mm: Define coherent device memory node
  mm: Add specialized fallback zonelist for coherent device memory nodes
  mm: Isolate coherent device memory nodes from HugeTLB allocation paths
  mm: Accommodate coherent device memory nodes in MPOL_BIND implementation
  mm: Add new flag VM_CDM for coherent device memory
  mm: Make VM_CDM marked VMAs non migratable
  mm: Add a new migration function migrate_virtual_range()
  mm: Add N_COHERENT_DEVICE node type into node_states[]

 Documentation/ABI/stable/sysfs-devices-node |  7 +++
 drivers/base/node.c                         |  6 +++
 include/linux/mempolicy.h                   | 24 +++++++++
 include/linux/migrate.h                     |  3 ++
 include/linux/mm.h                          |  5 ++
 include/linux/mmzone.h                      | 29 ++++++++++
 include/linux/nodemask.h                    |  3 ++
 mm/Kconfig                                  | 13 +++++
 mm/hugetlb.c                                | 38 ++++++++++++-
 mm/memory_hotplug.c                         | 10 ++++
 mm/mempolicy.c                              | 70 ++++++++++++++++++++++--
 mm/migrate.c                                | 84 +++++++++++++++++++++++++++++
 mm/page_alloc.c                             | 10 ++++
 13 files changed, 295 insertions(+), 7 deletions(-)

-- 
2.1.0