mm,thp,rmap: handle the normal !PageCompound case first

Commit ("mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts")
propagated the "if (compound) {lock} else if (PageCompound) {lock} else
{atomic}" pattern throughout; but Linus hated the way that gives primacy
to the uncommon case: switch to "if (!PageCompound) {atomic} else if
(compound) {lock} else {lock}" throughout.  Linus has a bigger idea for
how to improve it all, but here just make that rearrangement.

Link: https://lkml.kernel.org/r/fca2f694-2098-b0ef-d4e-f1d8b94d318c@google.comSigned-off-by: Hugh Dickins <hughd@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/rmap.c

@@ -1311,7 +1311,11 @@ void page_add_anon_rmap(struct page *page,
 	else
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
 
-	if (compound && PageTransHuge(page)) {
+	if (likely(!PageCompound(page))) {
+		first = atomic_inc_and_test(&page->_mapcount);
+		nr = first;
+
+	} else if (compound && PageTransHuge(page)) {
 		lock_compound_mapcounts(page, &mapcounts);
 		first = !mapcounts.compound_mapcount;
 		mapcounts.compound_mapcount++;
@@ -1321,8 +1325,7 @@ void page_add_anon_rmap(struct page *page,
 				nr = nr_subpages_unmapped(page, nr_pmdmapped);
 		}
 		unlock_compound_mapcounts(page, &mapcounts);
-
-	} else if (PageCompound(page)) {
+	} else {
 		struct page *head = compound_head(page);
 
 		lock_compound_mapcounts(head, &mapcounts);
@@ -1330,10 +1333,6 @@ void page_add_anon_rmap(struct page *page,
 		first = subpage_mapcount_inc(page);
 		nr = first && !mapcounts.compound_mapcount;
 		unlock_compound_mapcounts(head, &mapcounts);
-
-	} else {
-		first = atomic_inc_and_test(&page->_mapcount);
-		nr = first;
 	}
 
 	VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
@@ -1373,20 +1372,23 @@ void page_add_anon_rmap(struct page *page,
 void page_add_new_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address)
 {
-	const bool compound = PageCompound(page);
-	int nr = compound ? thp_nr_pages(page) : 1;
+	int nr;
 
 	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
 	__SetPageSwapBacked(page);
-	if (compound) {
+
+	if (likely(!PageCompound(page))) {
+		/* increment count (starts at -1) */
+		atomic_set(&page->_mapcount, 0);
+		nr = 1;
+	} else {
 		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 		/* increment count (starts at -1) */
 		atomic_set(compound_mapcount_ptr(page), 0);
+		nr = thp_nr_pages(page);
 		__mod_lruvec_page_state(page, NR_ANON_THPS, nr);
-	} else {
-		/* increment count (starts at -1) */
-		atomic_set(&page->_mapcount, 0);
 	}
+
 	__mod_lruvec_page_state(page, NR_ANON_MAPPED, nr);
 	__page_set_anon_rmap(page, vma, address, 1);
 }
@@ -1409,7 +1411,11 @@ void page_add_file_rmap(struct page *page,
 	VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
 	lock_page_memcg(page);
 
-	if (compound && PageTransHuge(page)) {
+	if (likely(!PageCompound(page))) {
+		first = atomic_inc_and_test(&page->_mapcount);
+		nr = first;
+
+	} else if (compound && PageTransHuge(page)) {
 		lock_compound_mapcounts(page, &mapcounts);
 		first = !mapcounts.compound_mapcount;
 		mapcounts.compound_mapcount++;
@@ -1419,8 +1425,7 @@ void page_add_file_rmap(struct page *page,
 				nr = nr_subpages_unmapped(page, nr_pmdmapped);
 		}
 		unlock_compound_mapcounts(page, &mapcounts);
-
-	} else if (PageCompound(page)) {
+	} else {
 		struct page *head = compound_head(page);
 
 		lock_compound_mapcounts(head, &mapcounts);
@@ -1428,10 +1433,6 @@ void page_add_file_rmap(struct page *page,
 		first = subpage_mapcount_inc(page);
 		nr = first && !mapcounts.compound_mapcount;
 		unlock_compound_mapcounts(head, &mapcounts);
-
-	} else {
-		first = atomic_inc_and_test(&page->_mapcount);
-		nr = first;
 	}
 
 	if (nr_pmdmapped)
@@ -1471,7 +1472,11 @@ void page_remove_rmap(struct page *page,
 	lock_page_memcg(page);
 
 	/* page still mapped by someone else? */
-	if (compound && PageTransHuge(page)) {
+	if (likely(!PageCompound(page))) {
+		last = atomic_add_negative(-1, &page->_mapcount);
+		nr = last;
+
+	} else if (compound && PageTransHuge(page)) {
 		lock_compound_mapcounts(page, &mapcounts);
 		mapcounts.compound_mapcount--;
 		last = !mapcounts.compound_mapcount;
@@ -1481,8 +1486,7 @@ void page_remove_rmap(struct page *page,
 				nr = nr_subpages_unmapped(page, nr_pmdmapped);
 		}
 		unlock_compound_mapcounts(page, &mapcounts);
-
-	} else if (PageCompound(page)) {
+	} else {
 		struct page *head = compound_head(page);
 
 		lock_compound_mapcounts(head, &mapcounts);
@@ -1490,10 +1494,6 @@ void page_remove_rmap(struct page *page,
 		last = subpage_mapcount_dec(page);
 		nr = last && !mapcounts.compound_mapcount;
 		unlock_compound_mapcounts(head, &mapcounts);
-
-	} else {
-		last = atomic_add_negative(-1, &page->_mapcount);
-		nr = last;
 	}
 
 	if (nr_pmdmapped) {