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mm/memory_hotplug: support memmap_on_memory when memmap is not aligned to pageblocks

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Currently, memmap_on_memory feature is only supported with memory block
sizes that result in vmemmap pages covering full page blocks.  This is
because memory onlining/offlining code requires applicable ranges to be
pageblock-aligned, for example, to set the migratetypes properly.

This patch helps to lift that restriction by reserving more pages than
required for vmemmap space.  This helps the start address to be page block
aligned with different memory block sizes.  Using this facility implies
the kernel will be reserving some pages for every memoryblock.  This
allows the memmap on memory feature to be widely useful with different
memory block size values.

For ex: with 64K page size and 256MiB memory block size, we require 4
pages to map vmemmap pages, To align things correctly we end up adding a
reserve of 28 pages.  ie, for every 4096 pages 28 pages get reserved.

Link: https://lkml.kernel.org/r/20230808091501.287660-5-aneesh.kumar@linux.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Aneesh Kumar K.V
2023-08-08 14:44:59 +05:30
committed by Andrew Morton
parent 85a2b4b08f
commit 2d1f649c7c
2 changed files with 113 additions and 19 deletions
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Documentation/admin-guide/mm/memory-hotplug.rst
+12
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@@ -433,6 +433,18 @@ The following module parameters are currently defined:
memory in a way that huge pages in bigger
granularity cannot be formed on hotplugged
memory.
With value "force" it could result in memory
wastage due to memmap size limitations. For
example, if the memmap for a memory block
requires 1 MiB, but the pageblock size is 2
MiB, 1 MiB of hotplugged memory will be wasted.
Note that there are still cases where the
feature cannot be enforced: for example, if the
memmap is smaller than a single page, or if the
architecture does not support the forced mode
in all configurations.
``online_policy`` read-write: Set the basic policy used for
automatic zone selection when onlining memory
blocks without specifying a target zone.