[RFC PATCH 0/4] mm, memory_hotplug: allocate memmap from hotadded memory

[Oscar Salvador]

Hi,

this patchset is based on Michal's patchset [1].
Patch#1, patch#2 and patch#4 are quite the same.
They just needed little changes to adapt it to current codestream,
so it seemed fair to leave them.

---------
Original cover:

This is another step to make the memory hotplug more usable. The primary
goal of this patchset is to reduce memory overhead of the hot added
memory (at least for SPARSE_VMEMMAP memory model). Currently we use
kmalloc to poppulate memmap (struct page array) which has two main
drawbacks a) it consumes an additional memory until the hotadded memory
itslef is onlined and b) memmap might end up on a different numa node
which is especially true for movable_node configuration.

a) is problem especially for memory hotplug based memory "ballooning"
solutions when the delay between physical memory hotplug and the
onlining can lead to OOM and that led to introduction of hacks like auto
onlining (see 31bc3858ea3e ("memory-hotplug: add automatic onlining
policy for the newly added memory")).
b) can have performance drawbacks.

One way to mitigate both issues is to simply allocate memmap array
(which is the largest memory footprint of the physical memory hotplug)
from the hotadded memory itself. VMEMMAP memory model allows us to map
any pfn range so the memory doesn't need to be online to be usable
for the array. See patch 3 for more details. In short I am reusing an
existing vmem_altmap which wants to achieve the same thing for nvdim
device memory.

There is also one potential drawback, though. If somebody uses memory
hotplug for 1G (gigantic) hugetlb pages then this scheme will not work
for them obviously because each memory block will contain reserved
area. Large x86 machines will use 2G memblocks so at least one 1G page
will be available but this is still not 2G...

I am not really sure somebody does that and how reliable that can work
actually. Nevertheless, I _believe_ that onlining more memory into
virtual machines is much more common usecase. Anyway if there ever is a
strong demand for such a usecase we have basically 3 options a) enlarge
memory blocks even more b) enhance altmap allocation strategy and reuse
low memory sections to host memmaps of other sections on the same NUMA
node c) have the memmap allocation strategy configurable to fallback to
the current allocation.

---------

Old version of this patchset would blow up because we were clearing the
pmds while we still had to reference pages backed by that memory.
I picked another approach which does not force us to touch arch specific code
in that regard.

Overall design:

With the preface of:

    1) Whenever you hot-add a range, this is the same range that will be hot-removed.
       This is just because you can't remove half of a DIMM, in the same way you can't
       remove half of a device in qemu.
       A device/DIMM are added/removed as a whole.

    2) Every add_memory()->add_memory_resource()->arch_add_memory()->__add_pages()
       will use a new altmap because it is a different hot-added range.

    3) When you hot-remove a range, the sections will be removed sequantially
       starting from the first section of the range and ending with the last one.

    4) hot-remove operations are protected by hotplug lock, so no parallel operations
       can take place.

    The current design is as follows:

    hot-remove operation)

    - __kfree_section_memmap will be called for every section to be removed.
    - We catch the first vmemmap_page and we pin it to a global variable.
    - Further calls to __kfree_section_memmap will decrease refcount of
      the vmemmap page without calling vmemmap_free().
      We defer the call to vmemmap_free() untill all sections are removed
    - If the refcount drops to 0, we know that we hit the last section.
    - We clear the global variable.
    - We call vmemmap_free for [last_section, current_vmemmap_page)

    In case we are hot-removing a range that used altmap, the call to
    vmemmap_free must be done backwards, because the beginning of memory
    is used for the pagetables.
    Doing it this way, we ensure that by the time we remove the pagetables,
    those pages will not have to be referenced anymore.

    An example:

    (qemu) object_add memory-backend-ram,id=ram0,size=10G
    (qemu) device_add pc-dimm,id=dimm0,memdev=ram0,node=1

    - This has added: ffffea0004000000 - ffffea000427ffc0 (refcount: 80)

    When refcount of ffffea0004000000 drops to 0, vmemmap_free()
    will be called in this way:

    vmemmap_free: start/end: ffffea000de00000 - ffffea000e000000
    vmemmap_free: start/end: ffffea000dc00000 - ffffea000de00000
    vmemmap_free: start/end: ffffea000da00000 - ffffea000dc00000
    vmemmap_free: start/end: ffffea000d800000 - ffffea000da00000
    vmemmap_free: start/end: ffffea000d600000 - ffffea000d800000
    vmemmap_free: start/end: ffffea000d400000 - ffffea000d600000
    vmemmap_free: start/end: ffffea000d200000 - ffffea000d400000
    vmemmap_free: start/end: ffffea000d000000 - ffffea000d200000
    vmemmap_free: start/end: ffffea000ce00000 - ffffea000d000000
    vmemmap_free: start/end: ffffea000cc00000 - ffffea000ce00000
    vmemmap_free: start/end: ffffea000ca00000 - ffffea000cc00000
    vmemmap_free: start/end: ffffea000c800000 - ffffea000ca00000
    vmemmap_free: start/end: ffffea000c600000 - ffffea000c800000
    vmemmap_free: start/end: ffffea000c400000 - ffffea000c600000
    vmemmap_free: start/end: ffffea000c200000 - ffffea000c400000
    vmemmap_free: start/end: ffffea000c000000 - ffffea000c200000
    vmemmap_free: start/end: ffffea000be00000 - ffffea000c000000
    ...
    ...
    vmemmap_free: start/end: ffffea0004000000 - ffffea0004200000


    [Testing]

    - Tested ony on x86_64
    - Several tests were carried out with memblocks of different sizes.
    - Tests were performed adding different memory-range sizes
      from 512M to 60GB.

    [Todo]
    - Look into hotplug gigantic pages case

Before investing more effort, I would like to hear some opinions/thoughts/ideas.

[1] https://lore.kernel.org/lkml/20170801124111.28881-1-mhocko@xxxxxxxxxx/

Michal Hocko (3):
  mm, memory_hotplug: cleanup memory offline path
  mm, memory_hotplug: provide a more generic restrictions for memory
    hotplug
  mm, sparse: rename kmalloc_section_memmap, __kfree_section_memmap

Oscar Salvador (1):
  mm, memory_hotplug: allocate memmap from the added memory range for
    sparse-vmemmap

 arch/arm64/mm/mmu.c            |   5 +-
 arch/ia64/mm/init.c            |   5 +-
 arch/powerpc/mm/init_64.c      |   2 +
 arch/powerpc/mm/mem.c          |   6 +-
 arch/s390/mm/init.c            |  12 +++-
 arch/sh/mm/init.c              |   6 +-
 arch/x86/mm/init_32.c          |   6 +-
 arch/x86/mm/init_64.c          |  17 ++++--
 include/linux/memory_hotplug.h |  35 ++++++++---
 include/linux/memremap.h       |  65 +++++++++++++++++++-
 include/linux/page-flags.h     |  18 ++++++
 kernel/memremap.c              |  12 ++--
 mm/compaction.c                |   3 +
 mm/hmm.c                       |   6 +-
 mm/memory_hotplug.c            | 133 ++++++++++++++++++++++++++++-------------
 mm/page_alloc.c                |  33 ++++++++--
 mm/page_isolation.c            |  13 +++-
 mm/sparse.c                    |  62 ++++++++++++++++---
 18 files changed, 345 insertions(+), 94 deletions(-)

-- 
2.13.6