mm: hugetlb_vmemmap: introduce the name HVO

It it inconvenient to mention the feature of optimizing vmemmap pages
associated with HugeTLB pages when communicating with others since there
is no specific or abbreviated name for it when it is first introduced.
Let us give it a name HVO (HugeTLB Vmemmap Optimization) from now.

This commit also updates the document about "hugetlb_free_vmemmap" by the
way discussed in thread [1].

Link: https://lore.kernel.org/all/[e-mail]/ [1]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muchun Song <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Reviewed-by: Mike Kravetz <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Xiongchun Duan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 	hugetlb_free_vmemmap=
 			[KNL] Reguires CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 			enabled.
+			Control if HugeTLB Vmemmap Optimization (HVO) is enabled.
 			Allows heavy hugetlb users to free up some more
 			memory (7 * PAGE_SIZE for each 2MB hugetlb page).
-			Format: { [oO][Nn]/Y/y/1 | [oO][Ff]/N/n/0 (default) }
+			Format: { on | off (default) }
 
-			[oO][Nn]/Y/y/1: enable the feature
-			[oO][Ff]/N/n/0: disable the feature
+			on: enable HVO
+			off: disable HVO
 
 			Built with CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON=y,
 			the default is on.

Documentation/admin-guide/mm/hugetlbpage.rst

···
 	will all result in 256 2M huge pages being allocated.  Valid default
 	huge page size is architecture dependent.
 hugetlb_free_vmemmap
-	When CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is set, this enables optimizing
-	unused vmemmap pages associated with each HugeTLB page.
+	When CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is set, this enables HugeTLB
+	Vmemmap Optimization (HVO).
 
 When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
 indicates the current number of pre-allocated huge pages of the default size.

Documentation/admin-guide/mm/memory-hotplug.rst

···
 - Concurrent activity that operates on the same physical memory area, such as
   allocating gigantic pages, can result in temporary offlining failures.
 
-- Out of memory when dissolving huge pages, especially when freeing unused
-  vmemmap pages associated with each hugetlb page is enabled.
+- Out of memory when dissolving huge pages, especially when HugeTLB Vmemmap
+  Optimization (HVO) is enabled.
 
   Offlining code may be able to migrate huge page contents, but may not be able
   to dissolve the source huge page because it fails allocating (unmovable) pages

Documentation/admin-guide/sysctl/vm.rst

···
 in include/linux/mm_types.h) is not power of two (an unusual system config could
 result in this).
 
-Enable (set to 1) or disable (set to 0) the feature of optimizing vmemmap pages
-associated with each HugeTLB page.
+Enable (set to 1) or disable (set to 0) HugeTLB Vmemmap Optimization (HVO).
 
 Once enabled, the vmemmap pages of subsequent allocation of HugeTLB pages from
 buddy allocator will be optimized (7 pages per 2MB HugeTLB page and 4095 pages

Documentation/mm/vmemmap_dedup.rst

···
 HugeTLB
 =======
 
+This section is to explain how HugeTLB Vmemmap Optimization (HVO) works.
+
 The struct page structures (page structs) are used to describe a physical
 page frame. By default, there is a one-to-one mapping from a page frame to
 it's corresponding page struct.

fs/Kconfig

···
 
 #
 # Select this config option from the architecture Kconfig, if it is preferred
-# to enable the feature of minimizing overhead of struct page associated with
-# each HugeTLB page.
+# to enable the feature of HugeTLB Vmemmap Optimization (HVO).
 #
 config ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 	bool
···
 	depends on SPARSEMEM_VMEMMAP
 
 config HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON
-	bool "Default optimizing vmemmap pages of HugeTLB to on"
+	bool "HugeTLB Vmemmap Optimization (HVO) defaults to on"
 	default n
 	depends on HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 	help
-	  When using HUGETLB_PAGE_OPTIMIZE_VMEMMAP, the optimizing unused vmemmap
-	  pages associated with each HugeTLB page is default off. Say Y here
-	  to enable optimizing vmemmap pages of HugeTLB by default. It can then
-	  be disabled on the command line via hugetlb_free_vmemmap=off.
+	  The HugeTLB VmemmapvOptimization (HVO) defaults to off. Say Y here to
+	  enable HVO by default. It can be disabled via hugetlb_free_vmemmap=off
+	  (boot command line) or hugetlb_optimize_vmemmap (sysctl).
 
 config MEMFD_CREATE
 	def_bool TMPFS || HUGETLBFS

include/linux/page-flags.h

···
 DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 
 /*
- * If the feature of optimizing vmemmap pages associated with each HugeTLB
- * page is enabled, the head vmemmap page frame is reused and all of the tail
+ * If HVO is enabled, the head vmemmap page frame is reused and all of the tail
  * vmemmap addresses map to the head vmemmap page frame (furture details can
  * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
  * words, there are more than one page struct with PG_head associated with each

mm/hugetlb_vmemmap.c

···
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Optimize vmemmap pages associated with HugeTLB
+ * HugeTLB Vmemmap Optimization (HVO)
  *
- * Copyright (c) 2020, Bytedance. All rights reserved.
+ * Copyright (c) 2020, ByteDance. All rights reserved.
  *
  *     Author: Muchun Song <[e-mail]>
  *
···
 
 	/*
 	 * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
-	 * page structs that can be used when CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP,
-	 * so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
+	 * page structs that can be used when HVO is enabled, add a BUILD_BUG_ON
+	 * to catch invalid usage of the tail page structs.
 	 */
 	BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));

mm/hugetlb_vmemmap.h

···
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Optimize vmemmap pages associated with HugeTLB
+ * HugeTLB Vmemmap Optimization (HVO)
  *
- * Copyright (c) 2020, Bytedance. All rights reserved.
+ * Copyright (c) 2020, ByteDance. All rights reserved.
  *
  *     Author: Muchun Song <[e-mail]>
  */