[RFC PATCH] mm, hugetlb: implement movable_gigantic_pages sysctl

[Gregory Price <gourry@gourry.net>]

This reintroduces a concept removed by
commit d6cb41cc44c6 ("mm, hugetlb: remove hugepages_treat_as_movable sysctl")

This sysctl provides some flexibility between multiple requirements which
are difficult to square without adding significantly more complexity.

1) onlining memory in ZONE_MOVABLE to maintain hotplug compatibility
2) onlining memory in ZONE_MOVABLE to increase reliability of hugepage
   allocation.

When the user's intent for ZONE_MOVABLE is to allow more reliable huge
page allocation (as opposed to enabling hotplugability), disallowing 1GB
hugepages in this region this region is pointless.  So if hotplug is not
a requirement, we can loosen the restrictions to allow 1GB gigantic pages
in ZONE_MOVABLE.

Since 1GB can be difficult to migrate / has impacts on compaction /
defragmentation, we don't enable this by default.  However, since there
are scenarios where gigantic pages are migratable (hugetlb available in
multiple places), we should allow use of these on zone movable regions.

Note: Boot-time CMA is not possible for driver-managed hotplug memory,
as CMA requires the memory to be registered as SystemRAM at boot time.

Cc: David Hildenbrand <david@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Alexandru Moise <00moses.alexander00@gmail.com>
Suggested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://lore.kernel.org/all/20180201193132.Hk7vI_xaU%25akpm@linux-foundation.org/
---
 Documentation/admin-guide/sysctl/vm.rst | 17 +++++++++++++++++
 include/linux/hugetlb.h                 |  3 ++-
 mm/hugetlb.c                            | 11 +++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 4d71211fdad8..89dcee3c3239 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -54,6 +54,7 @@ Currently, these files are in /proc/sys/vm:
 - mmap_min_addr
 - mmap_rnd_bits
 - mmap_rnd_compat_bits
+- movable_gigantic_pages
 - nr_hugepages
 - nr_hugepages_mempolicy
 - nr_overcommit_hugepages
@@ -624,6 +625,22 @@ This value can be changed after boot using the
 /proc/sys/vm/mmap_rnd_compat_bits tunable
 
 
+movable_gigantic_pages
+======================
+
+This parameter controls whether gigantic pages may be allocated from
+ZONE_MOVABLE. If set to non-zero, gigantic hugepages can be allocated
+from ZONE_MOVABLE. ZONE_MOVABLE memory may be created via the kernel
+boot parameter `kernelcore` or via memory hotplug as discussed in
+Documentation/admin-guide/mm/memory-hotplug.rst.
+
+Support may depend on specific architecture.
+
+Note that using ZONE_MOVABLE gigantic pages may make features like
+memory hotremove more unreliable, as migrating gigantic pages is more
+difficult due to needing larger amounts of phyiscally contiguous memory.
+
+
 nr_hugepages
 ============
 
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 526d27e88b3b..38870d21724a 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -172,6 +172,7 @@ bool hugetlbfs_pagecache_present(struct hstate *h,
 
 struct address_space *hugetlb_folio_mapping_lock_write(struct folio *folio);
 
+extern int movable_gigantic_pages __read_mostly;
 extern int sysctl_hugetlb_shm_group;
 extern struct list_head huge_boot_pages[MAX_NUMNODES];
 
@@ -916,7 +917,7 @@ static inline bool hugepage_movable_supported(struct hstate *h)
 	if (!hugepage_migration_supported(h))
 		return false;
 
-	if (hstate_is_gigantic(h))
+	if (hstate_is_gigantic(h) && !movable_gigantic_pages)
 		return false;
 	return true;
 }
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 753f99b4c718..24dbd30d1b69 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -55,6 +55,8 @@
 #include "hugetlb_cma.h"
 #include <linux/page-isolation.h>
 
+int movable_gigantic_pages;
+
 int hugetlb_max_hstate __read_mostly;
 unsigned int default_hstate_idx;
 struct hstate hstates[HUGE_MAX_HSTATE];
@@ -5195,6 +5197,15 @@ static const struct ctl_table hugetlb_table[] = {
 		.mode		= 0644,
 		.proc_handler	= hugetlb_overcommit_handler,
 	},
+#ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
+	{
+		.procname	= "movable_gigantic_pages",
+		.data		= &movable_gigantic_pages,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
+#endif
 };
 
 static void __init hugetlb_sysctl_init(void)
-- 
2.51.0