mm/util: reduce stack usage of folio_mapcount

From: Kairui Song <[email protected]>

folio_entire_mapcount will call PageHeadHuge which is a function call, and blocks the compiler from recognizing this redundant load.

After rearranging the code, stack usage is dropped from 32 to 24, and the function size is smaller (tested on GCC 12):

Before:
Stack usage:
mm/util.c:845:5:folio_mapcount 32 static
Size:
0000000000000ea0 00000000000000c7 T folio_mapcount

After:
Stack usage:
mm/util.c:845:5:folio_mapcount 24 static
Size:
0000000000000ea0 00000000000000b0 T folio_mapcount

...

mm/util.c +1 -1

@@ -850,10 +850,10 @@ int folio_mapcount(struct folio *folio)
 		return atomic_read(&folio->_mapcount) + 1;
 
 	compound = folio_entire_mapcount(folio);
-	nr = folio_nr_pages(folio);
 	if (folio_test_hugetlb(folio))
 		return compound;
 	ret = compound;
+	nr = folio_nr_pages(folio);
 	for (i = 0; i < nr; i++)
 		ret += atomic_read(&folio_page(folio, i)->_mapcount) + 1;
 	/* File pages has compound_mapcount included in _mapcount */

Signature · 1 line

2.35.2

Quoted 4 lines · On Tue, 2 Aug 2022 01:31:55 +0800 Kairui Song <[email protected]> wrote:

From: Kairui Song <[email protected]>

folio_entire_mapcount will call PageHeadHuge which is a function call, and blocks the compiler from recognizing this redundant load.

Did you mean folio_test_hugetlb() rather than folio_entire_mapcount()?

Quoted 22 lines

After rearranging the code, stack usage is dropped from 32 to 24, and the function size is smaller (tested on GCC 12):

Before:
Stack usage:
mm/util.c:845:5:folio_mapcount 32 static
Size:
0000000000000ea0 00000000000000c7 T folio_mapcount

After:
Stack usage:
mm/util.c:845:5:folio_mapcount 24 static
Size:
0000000000000ea0 00000000000000b0 T folio_mapcount

...

mm/util.c +0 -0

@@ -850,10 +850,10 @@ int folio_mapcount(struct folio *folio)
              return atomic_read(&folio->_mapcount) + 1;
 

Quoted 11 lines

mm/util.c +1 -2

      compound = folio_entire_mapcount(folio);
-     nr = folio_nr_pages(folio);
      if (folio_test_hugetlb(folio))
              return compound;
      ret = compound;
+     nr = folio_nr_pages(folio);
      for (i = 0; i < nr; i++)
              ret += atomic_read(&folio_page(folio, i)->_mapcount) + 1;
      /* File pages has compound_mapcount included in _mapcount */
--

2.35.2