Re: [PATCH v3 3/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory

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On 04.03.21 11:00, Oscar Salvador wrote:
Self stored memmap leads to a sparse memory situation which is unsuitable
for workloads that requires large contiguous memory chunks, so make this
an opt-in which needs to be explicitly enabled.

To control this, let memory_hotplug have its own memory space, as suggested
by David, so we can add memory_hotplug.memmap_on_memory parameter.

Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
  Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
  mm/Makefile                                     |  5 ++++-
  mm/memory_hotplug.c                             |  8 +++++++-
  3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 04545725f187..e626dab39c60 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2794,6 +2794,20 @@
  			seconds.  Use this parameter to check at some
  			other rate.  0 disables periodic checking.
+ memory_hotplug.memmap_on_memory
+			[KNL,X86,ARM] Boolean flag to enable this feature.

Right now it can be set on any arch with memory hotplug, right? It's simply not effective.

+			Format: {on | off (default)}
+			When enabled, memory to build the pages tables for the
+			memmap array describing the hot-added range will be taken
+			from the range itself, so the memmap page tables will be
+			self-hosted.
+			Since only single memory device ranges are supported at
+			the moment, this option is disabled by default because
+			it might have an impact on workloads that needs large
+			contiguous memory chunks.
+			The state of the flag can be read in
+			/sys/module/memory_hotplug/parameters/memmap_on_memory.

Maybe want to add that even if enabled, there are cases where it is not effective?

+
  	memtest=	[KNL,X86,ARM,PPC] Enable memtest
  			Format: <integer>
  			default : 0 <disable>
diff --git a/mm/Makefile b/mm/Makefile
index 72227b24a616..82ae9482f5e3 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -58,9 +58,13 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
  page-alloc-y := page_alloc.o
  page-alloc-$(CONFIG_SHUFFLE_PAGE_ALLOCATOR) += shuffle.o
+# Give 'memory_hotplug' its own module-parameter namespace
+memory-hotplug-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
+
  obj-y += page-alloc.o
  obj-y += init-mm.o
  obj-y += memblock.o
+obj-y += $(memory-hotplug-y)
ifdef CONFIG_MMU
  	obj-$(CONFIG_ADVISE_SYSCALLS)	+= madvise.o
@@ -83,7 +87,6 @@ obj-$(CONFIG_SLUB) += slub.o
  obj-$(CONFIG_KASAN)	+= kasan/
  obj-$(CONFIG_KFENCE) += kfence/
  obj-$(CONFIG_FAILSLAB) += failslab.o
-obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
  obj-$(CONFIG_MEMTEST)		+= memtest.o
  obj-$(CONFIG_MIGRATION) += migrate.o
  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 63e5a0e9a6f3..94b0ec3d2ff2 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -42,7 +42,13 @@
  #include "internal.h"
  #include "shuffle.h"
-static bool memmap_on_memory;
+
+/*
+ * memory_hotplug.memmap_on_memory parameter
+ */
+static bool memmap_on_memory __ro_after_init;
+module_param(memmap_on_memory, bool, 0444);
+MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug");

Wondering if this makes sense getting wrapped in

#ifdef CONFIG MHP_MEMMAP_ON_MEMORY

just a thought.

LGTM

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb





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