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Date:   Wed, 9 Nov 2022 18:18:49 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxcg.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>, Peter Xu <peterx@redhat.com>,
        Yang Shi <shy828301@gmail.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Naoya Horiguchi <naoya.horiguchi@linux.dev>,
        Mina Almasry <almasrymina@google.com>,
        James Houghton <jthoughton@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH 4/3] mm,thp,rmap: handle the normal !PageCompound case
 first
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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.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/rmap.c | 54 +++++++++++++++++++++++++++---------------------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 512e53cae2ca..4833d28c5e1a 100644
--- a/mm/rmap.c
+++ b/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) {
-- 
2.35.3