mm: hugetlb_vmemmap: improve hugetlb_vmemmap code readability

There is a discussion about the name of hugetlb_vmemmap_alloc/free in
thread [1].  The suggestion suggested by David is rename "alloc/free" to
"optimize/restore" to make functionalities clearer to users, "optimize"
means the function will optimize vmemmap pages, while "restore" means
restoring its vmemmap pages discared before.  This commit does this.

Another discussion is the confusion RESERVE_VMEMMAP_NR isn't used
explicitly for vmemmap_addr but implicitly for vmemmap_end in
hugetlb_vmemmap_alloc/free.  David suggested we can compute what
hugetlb_vmemmap_init() does now at runtime.  We do not need to worry for
the overhead of computing at runtime since the calculation is simple
enough and those functions are not in a hot path.  This commit has the
following improvements:

  1) The function suffixed name ("optimize/restore") is more expressive.
  2) The logic becomes less weird in hugetlb_vmemmap_optimize/restore().
  3) The hugetlb_vmemmap_init() does not need to be exported anymore.
  4) A ->optimize_vmemmap_pages field in struct hstate is killed.
  5) There is only one place where checks is_power_of_2(sizeof(struct
     page)) instead of two places.
  6) Add more comments for hugetlb_vmemmap_optimize/restore().
  7) For external users, hugetlb_optimize_vmemmap_pages() is used for
     detecting if the HugeTLB's vmemmap pages is optimizable originally.
     In this commit, it is killed and we introduce a new helper
     hugetlb_vmemmap_optimizable() to replace it.  The name is more
     expressive.

Link: https://lore.kernel.org/all/20220404074652.68024-2-songmuchun@bytedance.com/ [1]
Link: https://lkml.kernel.org/r/20220628092235.91270-7-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 28 insertions, 13 deletions

diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index ba66fadad9fc..25bd0e002431 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -11,35 +11,50 @@
 #include <linux/hugetlb.h>
 
 #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
-int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head);
-void hugetlb_vmemmap_free(struct hstate *h, struct page *head);
-void hugetlb_vmemmap_init(struct hstate *h);
+int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
+void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
 
 /*
- * How many vmemmap pages associated with a HugeTLB page that can be
- * optimized and freed to the buddy allocator.
+ * Reserve one vmemmap page, all vmemmap addresses are mapped to it. See
+ * Documentation/vm/vmemmap_dedup.rst.
  */
-static inline unsigned int hugetlb_optimize_vmemmap_pages(struct hstate *h)
+#define HUGETLB_VMEMMAP_RESERVE_SIZE	PAGE_SIZE
+
+static inline unsigned int hugetlb_vmemmap_size(const struct hstate *h)
 {
-	return h->optimize_vmemmap_pages;
+	return pages_per_huge_page(h) * sizeof(struct page);
+}
+
+/*
+ * Return how many vmemmap size associated with a HugeTLB page that can be
+ * optimized and can be freed to the buddy allocator.
+ */
+static inline unsigned int hugetlb_vmemmap_optimizable_size(const struct hstate *h)
+{
+	int size = hugetlb_vmemmap_size(h) - HUGETLB_VMEMMAP_RESERVE_SIZE;
+
+	if (!is_power_of_2(sizeof(struct page)))
+		return 0;
+	return size > 0 ? size : 0;
 }
 #else
-static inline int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
+static inline int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
 {
 	return 0;
 }
 
-static inline void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
+static inline void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
 {
 }
 
-static inline void hugetlb_vmemmap_init(struct hstate *h)
+static inline unsigned int hugetlb_vmemmap_optimizable_size(const struct hstate *h)
 {
+	return 0;
 }
+#endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */
 
-static inline unsigned int hugetlb_optimize_vmemmap_pages(struct hstate *h)
+static inline bool hugetlb_vmemmap_optimizable(const struct hstate *h)
 {
-	return 0;
+	return hugetlb_vmemmap_optimizable_size(h) != 0;
 }
-#endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */
 #endif /* _LINUX_HUGETLB_VMEMMAP_H */