mm/swap: comment all the ifdef in swapops.h

swapops.h contains quite a few layers of ifdef, some of the "else" and
"endif" doesn't get proper comment on the macro so it's hard to follow on
what are they referring to.  Add the comments.

Link: https://lkml.kernel.org/r/20220811161331.37055-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Suggested-by: Nadav Amit <nadav.amit@gmail.com>
Reviewed-by: Huang Ying <ying.huang@intel.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swapops.h

@@ -247,8 +247,8 @@ extern void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 #ifdef CONFIG_HUGETLB_PAGE
 extern void __migration_entry_wait_huge(pte_t *ptep, spinlock_t *ptl);
 extern void migration_entry_wait_huge(struct vm_area_struct *vma, pte_t *pte);
-#endif
-#else
+#endif	/* CONFIG_HUGETLB_PAGE */
+#else  /* CONFIG_MIGRATION */
 static inline swp_entry_t make_readable_migration_entry(pgoff_t offset)
 {
 	return swp_entry(0, 0);
@@ -276,7 +276,7 @@ static inline void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 #ifdef CONFIG_HUGETLB_PAGE
 static inline void __migration_entry_wait_huge(pte_t *ptep, spinlock_t *ptl) { }
 static inline void migration_entry_wait_huge(struct vm_area_struct *vma, pte_t *pte) { }
-#endif
+#endif	/* CONFIG_HUGETLB_PAGE */
 static inline int is_writable_migration_entry(swp_entry_t entry)
 {
 	return 0;
@@ -286,7 +286,7 @@ static inline int is_readable_migration_entry(swp_entry_t entry)
 	return 0;
 }
 
-#endif
+#endif	/* CONFIG_MIGRATION */
 
 typedef unsigned long pte_marker;
 
@@ -426,7 +426,7 @@ static inline int is_pmd_migration_entry(pmd_t pmd)
 {
 	return is_swap_pmd(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
 }
-#else
+#else  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
 static inline int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
 		struct page *page)
 {
@@ -455,7 +455,7 @@ static inline int is_pmd_migration_entry(pmd_t pmd)
 {
 	return 0;
 }
-#endif
+#endif  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
 
 #ifdef CONFIG_MEMORY_FAILURE
 
@@ -490,7 +490,7 @@ static inline void num_poisoned_pages_sub(long i)
 	atomic_long_sub(i, &num_poisoned_pages);
 }
 
-#else
+#else  /* CONFIG_MEMORY_FAILURE */
 
 static inline swp_entry_t make_hwpoison_entry(struct page *page)
 {
@@ -509,7 +509,7 @@ static inline void num_poisoned_pages_inc(void)
 static inline void num_poisoned_pages_sub(long i)
 {
 }
-#endif
+#endif  /* CONFIG_MEMORY_FAILURE */
 
 static inline int non_swap_entry(swp_entry_t entry)
 {