mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped

Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? 
Yes.  Not by atomic64_t or cmpxchg games, those get difficult on 32-bit;
but if we slightly abuse subpages_mapcount by additionally demanding that
one bit be set there when the compound page is PMD-mapped, then a cascade
of two atomic ops is able to maintain the stats without bit_spin_lock.

This is harder to reason about than when bit_spin_locked, but I believe
safe; and no drift in stats detected when testing.  When there are racing
removes and adds, of course the sequence of operations is less well-
defined; but each operation on subpages_mapcount is atomically good.  What
might be disastrous, is if subpages_mapcount could ever fleetingly appear
negative: but the pte lock (or pmd lock) these rmap functions are called
under, ensures that a last remove cannot race ahead of a first add.

Continue to make an exception for hugetlb (PageHuge) pages, though that
exception can be easily removed by a further commit if necessary: leave
subpages_mapcount 0, don't bother with COMPOUND_MAPPED in its case, just
carry on checking compound_mapcount too in folio_mapped(), page_mapped().

Evidence is that this way goes slightly faster than the previous
implementation in all cases (pmds after ptes now taking around 103ms); and
relieves us of worrying about contention on the bit_spin_lock.

Link: https://lkml.kernel.org/r/3978f3ca-5473-55a7-4e14-efea5968d892@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Carpenter <error27@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <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: Yu Zhao <yuzhao@google.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 25 insertions, 96 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index e813785da613..459dc1c44d8a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1085,38 +1085,6 @@ int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
 	return page_vma_mkclean_one(&pvmw);
 }
 
-struct compound_mapcounts {
-	unsigned int compound_mapcount;
-	unsigned int subpages_mapcount;
-};
-
-/*
- * lock_compound_mapcounts() first locks, then copies subpages_mapcount and
- * compound_mapcount from head[1].compound_mapcount and subpages_mapcount,
- * converting from struct page's internal representation to logical count
- * (that is, adding 1 to compound_mapcount to hide its offset by -1).
- */
-static void lock_compound_mapcounts(struct page *head,
-		struct compound_mapcounts *local)
-{
-	bit_spin_lock(PG_locked, &head[1].flags);
-	local->compound_mapcount = atomic_read(compound_mapcount_ptr(head)) + 1;
-	local->subpages_mapcount = atomic_read(subpages_mapcount_ptr(head));
-}
-
-/*
- * After caller has updated subpage._mapcount, local subpages_mapcount and
- * local compound_mapcount, as necessary, unlock_compound_mapcounts() converts
- * and copies them back to the compound head[1] fields, and then unlocks.
- */
-static void unlock_compound_mapcounts(struct page *head,
-		struct compound_mapcounts *local)
-{
-	atomic_set(compound_mapcount_ptr(head), local->compound_mapcount - 1);
-	atomic_set(subpages_mapcount_ptr(head), local->subpages_mapcount);
-	bit_spin_unlock(PG_locked, &head[1].flags);
-}
-
 int total_compound_mapcount(struct page *head)
 {
 	int mapcount = head_compound_mapcount(head);
@@ -1140,34 +1108,6 @@ int total_compound_mapcount(struct page *head)
 	return mapcount;
 }
 
-/*
- * page_dup_compound_rmap(), used when copying mm,
- * provides a simple example of using lock_ and unlock_compound_mapcounts().
- */
-void page_dup_compound_rmap(struct page *head)
-{
-	struct compound_mapcounts mapcounts;
-
-	/*
-	 * Hugetlb pages could use lock_compound_mapcounts(), like THPs do;
-	 * but at present they are still being managed by atomic operations:
-	 * which are likely to be somewhat faster, so don't rush to convert
-	 * them over without evaluating the effect.
-	 *
-	 * Note that hugetlb does not call page_add_file_rmap():
-	 * here is where hugetlb shared page mapcount is raised.
-	 */
-	if (PageHuge(head)) {
-		atomic_inc(compound_mapcount_ptr(head));
-	} else if (PageTransHuge(head)) {
-		/* That test is redundant: it's for safety or to optimize out */
-
-		lock_compound_mapcounts(head, &mapcounts);
-		mapcounts.compound_mapcount++;
-		unlock_compound_mapcounts(head, &mapcounts);
-	}
-}
-
 /**
  * page_move_anon_rmap - move a page to our anon_vma
  * @page:	the page to move to our anon_vma
@@ -1277,7 +1217,7 @@ static void __page_check_anon_rmap(struct page *page,
 void page_add_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address, rmap_t flags)
 {
-	struct compound_mapcounts mapcounts;
+	atomic_t *mapped;
 	int nr = 0, nr_pmdmapped = 0;
 	bool compound = flags & RMAP_COMPOUND;
 	bool first = true;
@@ -1290,24 +1230,20 @@ void page_add_anon_rmap(struct page *page,
 		first = atomic_inc_and_test(&page->_mapcount);
 		nr = first;
 		if (first && PageCompound(page)) {
-			struct page *head = compound_head(page);
-
-			lock_compound_mapcounts(head, &mapcounts);
-			mapcounts.subpages_mapcount++;
-			nr = !mapcounts.compound_mapcount;
-			unlock_compound_mapcounts(head, &mapcounts);
+			mapped = subpages_mapcount_ptr(compound_head(page));
+			nr = atomic_inc_return_relaxed(mapped);
+			nr = !(nr & COMPOUND_MAPPED);
 		}
 	} else if (PageTransHuge(page)) {
 		/* That test is redundant: it's for safety or to optimize out */
 
-		lock_compound_mapcounts(page, &mapcounts);
-		first = !mapcounts.compound_mapcount;
-		mapcounts.compound_mapcount++;
+		first = atomic_inc_and_test(compound_mapcount_ptr(page));
 		if (first) {
+			mapped = subpages_mapcount_ptr(page);
+			nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
 			nr_pmdmapped = thp_nr_pages(page);
-			nr = nr_pmdmapped - mapcounts.subpages_mapcount;
+			nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED);
 		}
-		unlock_compound_mapcounts(page, &mapcounts);
 	}
 
 	VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
@@ -1360,6 +1296,7 @@ void page_add_new_anon_rmap(struct page *page,
 		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 		/* increment count (starts at -1) */
 		atomic_set(compound_mapcount_ptr(page), 0);
+		atomic_set(subpages_mapcount_ptr(page), COMPOUND_MAPPED);
 		nr = thp_nr_pages(page);
 		__mod_lruvec_page_state(page, NR_ANON_THPS, nr);
 	}
@@ -1379,7 +1316,7 @@ void page_add_new_anon_rmap(struct page *page,
 void page_add_file_rmap(struct page *page,
 	struct vm_area_struct *vma, bool compound)
 {
-	struct compound_mapcounts mapcounts;
+	atomic_t *mapped;
 	int nr = 0, nr_pmdmapped = 0;
 	bool first;
 
@@ -1391,24 +1328,20 @@ void page_add_file_rmap(struct page *page,
 		first = atomic_inc_and_test(&page->_mapcount);
 		nr = first;
 		if (first && PageCompound(page)) {
-			struct page *head = compound_head(page);
-
-			lock_compound_mapcounts(head, &mapcounts);
-			mapcounts.subpages_mapcount++;
-			nr = !mapcounts.compound_mapcount;
-			unlock_compound_mapcounts(head, &mapcounts);
+			mapped = subpages_mapcount_ptr(compound_head(page));
+			nr = atomic_inc_return_relaxed(mapped);
+			nr = !(nr & COMPOUND_MAPPED);
 		}
 	} else if (PageTransHuge(page)) {
 		/* That test is redundant: it's for safety or to optimize out */
 
-		lock_compound_mapcounts(page, &mapcounts);
-		first = !mapcounts.compound_mapcount;
-		mapcounts.compound_mapcount++;
+		first = atomic_inc_and_test(compound_mapcount_ptr(page));
 		if (first) {
+			mapped = subpages_mapcount_ptr(page);
+			nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
 			nr_pmdmapped = thp_nr_pages(page);
-			nr = nr_pmdmapped - mapcounts.subpages_mapcount;
+			nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED);
 		}
-		unlock_compound_mapcounts(page, &mapcounts);
 	}
 
 	if (nr_pmdmapped)
@@ -1432,7 +1365,7 @@ void page_add_file_rmap(struct page *page,
 void page_remove_rmap(struct page *page,
 	struct vm_area_struct *vma, bool compound)
 {
-	struct compound_mapcounts mapcounts;
+	atomic_t *mapped;
 	int nr = 0, nr_pmdmapped = 0;
 	bool last;
 
@@ -1452,24 +1385,20 @@ void page_remove_rmap(struct page *page,
 		last = atomic_add_negative(-1, &page->_mapcount);
 		nr = last;
 		if (last && PageCompound(page)) {
-			struct page *head = compound_head(page);
-
-			lock_compound_mapcounts(head, &mapcounts);
-			mapcounts.subpages_mapcount--;
-			nr = !mapcounts.compound_mapcount;
-			unlock_compound_mapcounts(head, &mapcounts);
+			mapped = subpages_mapcount_ptr(compound_head(page));
+			nr = atomic_dec_return_relaxed(mapped);
+			nr = !(nr & COMPOUND_MAPPED);
 		}
 	} else if (PageTransHuge(page)) {
 		/* That test is redundant: it's for safety or to optimize out */
 
-		lock_compound_mapcounts(page, &mapcounts);
-		mapcounts.compound_mapcount--;
-		last = !mapcounts.compound_mapcount;
+		last = atomic_add_negative(-1, compound_mapcount_ptr(page));
 		if (last) {
+			mapped = subpages_mapcount_ptr(page);
+			nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped);
 			nr_pmdmapped = thp_nr_pages(page);
-			nr = nr_pmdmapped - mapcounts.subpages_mapcount;
+			nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED);
 		}
-		unlock_compound_mapcounts(page, &mapcounts);
 	}
 
 	if (nr_pmdmapped) {