Re: [PATCH v2 2/2] docs/mm: Physical Memory: add structure, introduction and nodes description

[Mike Rapoport <rppt@kernel.org>]

On Tue, Jan 10, 2023 at 05:54:10PM +0100, Michal Hocko wrote:
> On Tue 10-01-23 17:23:58, Mike Rapoport wrote:
> [...]
> > +* ``ZONE_DMA`` and ``ZONE_DMA32`` represent memory suitable for DMA by
> > +  peripheral devices that cannot access all of the addressable memory.
> 
> I think it would be better to not keep the historical DMA based menaning
> and teach that future developers. You can say something like
> 
> ZONE_DMA and ZONE_DMA32 have historically been used for memory suitable
> for DMA. For many years there are better more robust interfaces to
> get memory with DMA specific requirements (Documentation/core-api/dma-api.rst).

But even today ZONE_DMA(32) means that the memory is suitable for DMA. This
is nicely encapsulated with dma APIs and there should be no new GFP_DMA
users, but still memory outside ZONE_DMA is not suitable for DMA.
 
> > +  Depending on the architecture, either of these zone types or even they both
> > +  can be disabled at build time using ``CONFIG_ZONE_DMA`` and
> > +  ``CONFIG_ZONE_DMA32`` configuration options. Some 64-bit platforms may need
> > +  both zones as they support peripherals with different DMA addressing
> > +  limitations.
> > +
> > +* ``ZONE_NORMAL`` is for normal memory that can be accessed by the kernel all
> > +  the time. DMA operations can be performed on pages in this zone if the DMA
> > +  devices support transfers to all addressable memory. ``ZONE_NORMAL`` is
> > +  always enabled.
> > +
> > +* ``ZONE_HIGHMEM`` is the part of the physical memory that is not covered by a
> > +  permanent mapping in the kernel page tables. The memory in this zone is only
> > +  accessible to the kernel using temporary mappings. This zone is available
> > +  only on some 32-bit architectures and is enabled with ``CONFIG_HIGHMEM``.
> > +
> > +* ``ZONE_MOVABLE`` is for normal accessible memory, just like ``ZONE_NORMAL``.
> > +  The difference is that most pages in ``ZONE_MOVABLE`` are movable.
> 
> This is really confusing because those pages are not really movable. You
> cannot move a page itself. I guess you meant to say something like
> 
> The difference is that there are means to migrate memory via
> migrate_pages interface. A typical example would be a memory mapped to
> userspace which can be rellocate the underlying memory content and
> update page tables so that userspace doesn't notice the physical data
> placement has changed.
 
I agree that this sentence is a bit confusing, but there's a clarification
below. Also, I'd like to keep this at high level without going to the
details about how exactly the pages can be migrated.

> > That means
> > +  that while virtual addresses of these pages do not change, their content may
> > +  move between different physical pages. ``ZONE_MOVABLE`` is only enabled when
> > +  one of ``kernelcore``, ``movablecore`` and ``movable_node`` parameters is
> > +  present in the kernel command line. See :ref:`Page migration
> > +  <page_migration>` for additional details.
> 
> This is not really true. The movable zone can be also enabled by memory
> hotplug. In fact it is one of the more common usecases for the zone
> because memory hot remove largerly depends on memory to be migrated for
> offlining to succeed in most cases.

Right. How about this version of ZONE_MOVABLE description:

* ``ZONE_MOVABLE`` is for normal accessible memory, just like ``ZONE_NORMAL``.
  The difference is that the contents of most pages in ``ZONE_MOVABLE`` is
  movable. That means that while virtual addresses of these pages do not
  change, their content may move between different physical pages. Often
  ``ZONE_MOVABLE`` is populated during memory hotplug, but it may be
  also populated on boot using one of ``kernelcore``, ``movablecore`` and
  ``movable_node`` kernel command line parameters. See :ref:`Page migration
  <page_migration>` and :ref:`Memory Hot(Un)Plug <_admin_guide_memory_hotplug>`
  for additional details.
 
> > +* ``ZONE_DEVICE`` represents memory residing on devices such as PMEM and GPU.
> > +  It has different characteristics than RAM zone types and it exists to provide
> > +  :ref:`struct page <Pages>` and memory map services for device driver
> > +  identified physical address ranges. ``ZONE_DEVICE`` is enabled with
> > +  configuration option ``CONFIG_ZONE_DEVICE``.
> > +
> > +It is important to note that many kernel operations can only take place using
> > +``ZONE_NORMAL`` so it is the most performance critical zone. Zones are
> > +discussed further in Section :ref:`Zones <zones>`.
> > +
> > +The relation between node and zone extents is determined by the physical memory
> > +map reported by the firmware, architectural constraints for memory addressing
> > +and certain parameters in the kernel command line.
> > +
> > +For example, with 32-bit kernel on an x86 UMA machine with 2 Gbytes of RAM the
> > +entire memory will be on node 0 and there will be three zones: ``ZONE_DMA``,
> > +``ZONE_NORMAL`` and ``ZONE_HIGHMEM``::
> > +
> > +  0                                                            2G
> > +  +-------------------------------------------------------------+
> > +  |                            node 0                           |
> > +  +-------------------------------------------------------------+
> > +
> > +  0         16M                    896M                        2G
> > +  +----------+-----------------------+--------------------------+
> > +  | ZONE_DMA |      ZONE_NORMAL      |       ZONE_HIGHMEM       |
> > +  +----------+-----------------------+--------------------------+
> > +
> > +
> > +With a kernel built with ``ZONE_DMA`` disabled and ``ZONE_DMA32`` enabled and
> > +booted with ``movablecore=80%`` parameter on an arm64 machine with 16 Gbytes of
> > +RAM equally split between two nodes, there will be ``ZONE_DMA32``,
> > +``ZONE_NORMAL`` and ``ZONE_MOVABLE`` on node 0, and ``ZONE_NORMAL`` and
> > +``ZONE_MOVABLE`` on node 1::
> > +
> > +
> > +  1G                                9G                         17G
> > +  +--------------------------------+ +--------------------------+
> > +  |              node 0            | |          node 1          |
> > +  +--------------------------------+ +--------------------------+
> > +
> > +  1G       4G        4200M          9G          9320M          17G
> > +  +---------+----------+-----------+ +------------+-------------+
> > +  |  DMA32  |  NORMAL  |  MOVABLE  | |   NORMAL   |   MOVABLE   |
> > +  +---------+----------+-----------+ +------------+-------------+
> 
> I think it is useful to note that nodes and zones can overlap in the
> physical address range. It is not uncommong to interleave two nodes and
> it is also possible that memory holes are memory hotplugged into MOVABLE
> zone arbitrarily in the physical address range.

Hmm, not sure I understand what you mean by "overlap".
For interleaved nodes you mean that node 0 may span, say [0x0, 0x2000) and
[0x4000, 06000) and node 1 spans [0x2000, 0x4000) and [0x6000, 0x8000)?

And as for MOVABLE zone, you mean that it can appear between ranges of
NORMAL zone?
 
> Other than that looks good to me and thanks for taking care of filling
> up these gaps! This is highly appreciated.

Thanks!

I'd appreciate more inputs ;-)

> -- 
> Michal Hocko
> SUSE Labs

-- 
Sincerely yours,
Mike.