mm, hugetlb: implement movable_gigantic_pages sysctl

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

This sysctl provides flexibility between ZONE_MOVABLE use cases:
1) onlining memory in ZONE_MOVABLE to maintain hotplug compatibility
2) onlining memory in ZONE_MOVABLE to make hugepage allocate reliable

When ZONE_MOVABLE is used to make huge page allocation more reliable,
disallowing gigantic pages memory in this region is pointless.  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.  Notably, 1GB pages can
only be migrated if another 1GB page is available - so hot-unplug will
fail if such a page cannot be found.

However, since there are scenarios where gigantic pages are migratable, we
should allow use of these on movable regions.

When not valid 1GB is available for migration, hot-unplug will retry
indefinitely (or until interrupted).  For example:

  echo 0 > node0/hugepages/..-1GB/nr_hugepages  # clear node0 1GB pages
  echo 1 > node1/hugepages/..-1GB/nr_hugepages  # reserve node1 1GB page
  ./alloc_huge_node1 &    # Allocate a 1GB page on node1
  ./node1_offline  &      # attempt to offline all node1 memory
  echo 1 > node0/hugepages/..-1GB/nr_hugepages  # reserve node0 1GB page

In this example, node1_offline will block indefinitely until the final
step, when a node0 1GB page is made available.

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. 
Additionally, 1GB huge pages are not supported by THP.

Link: https://lkml.kernel.org/r/20251221125603.2364174-1-gourry@gourry.net
Signed-off-by: Gregory Price <gourry@gourry.net>
Suggested-by: David Rientjes <rientjes@google.com>
Link: https://lore.kernel.org/all/20180201193132.Hk7vI_xaU%25akpm@linux-foundation.org/
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
Cc: Gregory Price <gourry@gourry.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index ca6ebeb5171c..b98ccb5cb210 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -53,6 +53,7 @@ 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
@@ -620,6 +621,33 @@ 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 pages 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 make memory hotremove unreliable.
+
+Memory hot-remove operations will block indefinitely until the admin reserves
+sufficient gigantic pages to service migration requests associated with the
+memory offlining process.  As HugeTLB gigantic page reservation is a manual
+process (via `nodeN/hugepages/.../nr_hugepages` interfaces) this may not be
+obvious when just attempting to offline a block of memory.
+
+Additionally, as multiple gigantic pages may be reserved on a single block,
+it may appear that gigantic pages are available for migration when in reality
+they are in the process of being removed. For example if `memoryN` contains
+two gigantic pages, one reserved and one allocated, and an admin attempts to
+offline that block, this operations may hang indefinitely unless another
+reserved gigantic page is available on another block `memoryM`.
+
+
 nr_hugepages
 ============