[PATCH V3 1/5] mm/hugetlb: Distinguish between migratability and movability

[Anshuman Khandual <anshuman.khandual@arm.com>]

During huge page allocation it's migratability is checked to determine if
it should be placed under movable zones with GFP_HIGHUSER_MOVABLE. But the
movability aspect of the huge page could depend on other factors than just
migratability. Movability in itself is a distinct property which should not
be tied with migratability alone.

This differentiates these two and implements an enhanced movability check
which also considers huge page size to determine if it is feasible to be
placed under a movable zone. At present it just checks for gigantic pages
but going forward it can incorporate other enhanced checks.

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Suggested-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 include/linux/hugetlb.h | 30 ++++++++++++++++++++++++++++++
 mm/hugetlb.c            |  2 +-
 mm/migrate.c            |  2 +-
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 087fd5f4..1b858d7 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -506,6 +506,31 @@ static inline bool hugepage_migration_supported(struct hstate *h)
 #endif
 }
 
+/*
+ * Movability check is different as compared to migration check.
+ * It determines whether or not a huge page should be placed on
+ * movable zone or not. Movability of any huge page should be
+ * required only if huge page size is supported for migration.
+ * There wont be any reason for the huge page to be movable if
+ * it is not migratable to start with. Also the size of the huge
+ * page should be large enough to be placed under a movable zone
+ * and still feasible enough to be migratable. Just the presence
+ * in movable zone does not make the migration feasible.
+ *
+ * So even though large huge page sizes like the gigantic ones
+ * are migratable they should not be movable because its not
+ * feasible to migrate them from movable zone.
+ */
+static inline bool hugepage_movable_supported(struct hstate *h)
+{
+	if (!hugepage_migration_supported(h))
+		return false;
+
+	if (hstate_is_gigantic(h))
+		return false;
+	return true;
+}
+
 static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
 					   struct mm_struct *mm, pte_t *pte)
 {
@@ -602,6 +627,11 @@ static inline bool hugepage_migration_supported(struct hstate *h)
 	return false;
 }
 
+static inline bool hugepage_movable_supported(struct hstate *h)
+{
+	return false;
+}
+
 static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
 					   struct mm_struct *mm, pte_t *pte)
 {
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5c390f5..f810cf0 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -919,7 +919,7 @@ static struct page *dequeue_huge_page_nodemask(struct hstate *h, gfp_t gfp_mask,
 /* Movability of hugepages depends on migration support. */
 static inline gfp_t htlb_alloc_mask(struct hstate *h)
 {
-	if (hugepage_migration_supported(h))
+	if (hugepage_movable_supported(h))
 		return GFP_HIGHUSER_MOVABLE;
 	else
 		return GFP_HIGHUSER;
diff --git a/mm/migrate.c b/mm/migrate.c
index 84381b5..bfda9e4 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1272,7 +1272,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	struct anon_vma *anon_vma = NULL;
 
 	/*
-	 * Movability of hugepages depends on architectures and hugepage size.
+	 * Migratability of hugepages depends on architectures and their size.
 	 * This check is necessary because some callers of hugepage migration
 	 * like soft offline and memory hotremove don't walk through page
 	 * tables or check whether the hugepage is pmd-based or not before
-- 
2.7.4