[PATCH v4 7/7] mm/swap: Cache swap migration A/D bits support

[Peter Xu <peterx@xxxxxxxxxx>]

Introduce a variable swap_migration_ad_supported to cache whether the arch
supports swap migration A/D bits.

Here one thing to mention is that SWP_MIG_TOTAL_BITS will internally
reference the other macro MAX_PHYSMEM_BITS, which is a function call on
x86 (constant on all the rest of archs).

It's safe to reference it in swapfile_init() because when reaching here
we're already during initcalls level 4 so we must have initialized 5-level
pgtable for x86_64 (right after early_identify_cpu() finishes).

- start_kernel
  - setup_arch
    - early_cpu_init
      - get_cpu_cap --> fetch from CPUID (including X86_FEATURE_LA57)
      - early_identify_cpu --> clear X86_FEATURE_LA57 (if early lvl5 not enabled (USE_EARLY_PGTABLE_L5))
  - arch_call_rest_init
    - rest_init
      - kernel_init
        - kernel_init_freeable
          - do_basic_setup
            - do_initcalls --> calls swapfile_init() (initcall level 4)

This should slightly speed up the migration swap entry handlings.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 include/linux/swapfile.h | 2 ++
 include/linux/swapops.h  | 7 +------
 mm/swapfile.c            | 8 ++++++++
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
index 165e0bd04862..2fbcc9afd814 100644
--- a/include/linux/swapfile.h
+++ b/include/linux/swapfile.h
@@ -10,5 +10,7 @@ extern struct swap_info_struct *swap_info[];
 extern unsigned long generic_max_swapfile_size(void);
 /* Maximum swapfile size supported for the arch (not inclusive). */
 extern unsigned long swapfile_maximum_size;
+/* Whether swap migration entry supports storing A/D bits for the arch */
+extern bool swap_migration_ad_supported;
 
 #endif /* _LINUX_SWAPFILE_H */
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index f25b566643f1..dbf9df854124 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -301,13 +301,8 @@ static inline swp_entry_t make_writable_migration_entry(pgoff_t offset)
  */
 static inline bool migration_entry_supports_ad(void)
 {
-	/*
-	 * max_swapfile_size() returns the max supported swp-offset plus 1.
-	 * We can support the migration A/D bits iff the pfn swap entry has
-	 * the offset large enough to cover all of them (PFN, A & D bits).
-	 */
 #ifdef CONFIG_SWAP
-	return swapfile_maximum_size >= (1UL << SWP_MIG_TOTAL_BITS);
+	return swap_migration_ad_supported;
 #else  /* CONFIG_SWAP */
 	return false;
 #endif	/* CONFIG_SWAP */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3cc64399df44..263b19e693cf 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -64,6 +64,9 @@ EXPORT_SYMBOL_GPL(nr_swap_pages);
 long total_swap_pages;
 static int least_priority = -1;
 unsigned long swapfile_maximum_size;
+#ifdef CONFIG_MIGRATION
+bool swap_migration_ad_supported;
+#endif	/* CONFIG_MIGRATION */
 
 static const char Bad_file[] = "Bad swap file entry ";
 static const char Unused_file[] = "Unused swap file entry ";
@@ -3680,6 +3683,11 @@ static int __init swapfile_init(void)
 
 	swapfile_maximum_size = arch_max_swapfile_size();
 
+#ifdef CONFIG_MIGRATION
+	if (swapfile_maximum_size >= (1UL << SWP_MIG_TOTAL_BITS))
+		swap_migration_ad_supported = true;
+#endif	/* CONFIG_MIGRATION */
+
 	return 0;
 }
 subsys_initcall(swapfile_init);
-- 
2.32.0