[PATCH v3 0/5] Allocate memmap from hotadded memory

From: Oscar Salvador
Date: Thu Jul 25 2019 - 14:07:56 EST

Here we go with v3.

v3 -> v2:
* Rewrite about vmemmap pages handling.
Prior to this version, I was (ab)using hugepages fields
from struct page, while here I am officially adding a new
sub-page type with the fields I need.

While I am still not 100% if this the right decision, and while I
still see some gaining in having MHP_MEMMAP_{MEMBLOCK,DEVICE},
having only one flag ease the code.
If the user wants to allocate memmaps per memblock, it'll
have to call add_memory() variants with memory-block granularity.

If we happen to have a more clear usecase MHP_MEMMAP_MEMBLOCK
flag in the future, so user does not have to bother about the way
it calls add_memory() variants, but only pass a flag, we can add it.
Actually, I already had the code, so add it in the future is going to be

* Granularity check when hot-removing memory.
Just checking that the granularity is the same.


- x86_64: small and large memblocks (128MB, 1G and 2G)

So far, only acpi memory hotplug uses the new flag.
The other callers can be changed depending on their needs.


This is another step to make memory hotplug more usable. The primary
goal of this patchset is to reduce memory overhead of the hot-added
memory (at least for SPARSEMEM_VMEMMAP memory model). The current way we use
to populate memmap (struct page array) has two main drawbacks:

a) it consumes an additional memory until the hotadded memory itself is
onlined and
b) memmap might end up on a different numa node which is especially true
for movable_node configuration.

a) it is a 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 all these issues is to simply allocate memmap array
(which is the largest memory footprint of the physical memory hotplug)
from the hot-added memory itself. SPARSEMEM_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.
This feature is only usable when CONFIG_SPARSEMEM_VMEMMAP is set.

[Overall design]:

Implementation wise we reuse vmem_altmap infrastructure to override
the default allocator used by vmemap_populate. Once the memmap is
allocated we need a way to mark altmap pfns used for the allocation.
If MHP_MEMMAP_ON_MEMORY flag was passed, we set up the layout of the
altmap structure at the beginning of __add_pages(), and then we call

MHP_MEMMAP_ON_MEMORY flag parameter will specify to allocate memmaps
from the hot-added range.
If callers wants memmaps to be allocated per memory block, it will
have to call add_memory() variants in memory-block granularity
spanning the whole range, while if it wants to allocate memmaps
per whole memory range, just one call will do.

Want to add 384MB (3 sections, 3 memory-blocks)

add_memory(0x1000, size_memory_block);
add_memory(0x2000, size_memory_block);
add_memory(0x3000, size_memory_block);


add_memory(0x1000, size_memory_block * 3);

One thing worth mention is that vmemmap pages residing in movable memory is not a
show-stopper for that memory to be offlined/migrated away.
Vmemmap pages are just ignored in that case and they stick around until sections
referred by those vmemmap pages are hot-removed.

Oscar Salvador (5):
mm,memory_hotplug: Introduce MHP_MEMMAP_ON_MEMORY
mm: Introduce a new Vmemmap page-type
mm,sparse: Add SECTION_USE_VMEMMAP flag
mm,memory_hotplug: Allocate memmap from the added memory range for
mm,memory_hotplug: Allow userspace to enable/disable vmemmap

arch/powerpc/mm/init_64.c | 7 ++
arch/s390/mm/init.c | 6 ++
arch/x86/mm/init_64.c | 10 +++
drivers/acpi/acpi_memhotplug.c | 3 +-
drivers/base/memory.c | 35 +++++++++-
drivers/dax/kmem.c | 2 +-
drivers/hv/hv_balloon.c | 2 +-
drivers/s390/char/sclp_cmd.c | 2 +-
drivers/xen/balloon.c | 2 +-
include/linux/memory_hotplug.h | 37 ++++++++--
include/linux/memremap.h | 2 +-
include/linux/mm.h | 17 +++++
include/linux/mm_types.h | 5 ++
include/linux/mmzone.h | 8 ++-
include/linux/page-flags.h | 19 +++++
mm/compaction.c | 7 ++
mm/memory_hotplug.c | 153 +++++++++++++++++++++++++++++++++++++----
mm/page_alloc.c | 26 ++++++-
mm/page_isolation.c | 14 +++-
mm/sparse.c | 116 ++++++++++++++++++++++++++++++-
20 files changed, 441 insertions(+), 32 deletions(-)