mm/swap: cache swap migration A/D bits support

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Dave Hansen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

2 files changed, 3 insertions, 6 deletions

diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
index e2d11ae4e73d..7ed529a77c5b 100644
--- a/include/linux/swapfile.h
+++ b/include/linux/swapfile.h
@@ -12,5 +12,7 @@ unsigned long arch_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 027b4095e132..86b95ccb81bb 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 */