mm: track mapcount of large folios in single value

Let's track the mapcount of large folios in a single value.  The mapcount
of a large folio currently corresponds to the sum of the entire mapcount
and all page mapcounts.

This sum is what we actually want to know in folio_mapcount() and it is
also sufficient for implementing folio_mapped().

With PTE-mapped THP becoming more important and more widely used, we want
to avoid looping over all pages of a folio just to obtain the mapcount of
large folios.  The comment "In the common case, avoid the loop when no
pages mapped by PTE" in folio_total_mapcount() does no longer hold for
mTHP that are always mapped by PTE.

Further, we are planning on using folio_mapcount() more frequently, and
might even want to remove page mapcounts for large folios in some kernel
configs.  Therefore, allow for reading the mapcount of large folios
efficiently and atomically without looping over any pages.

Maintain the mapcount also for hugetlb pages for simplicity.  Use the new
mapcount to implement folio_mapcount() and folio_mapped().  Make
page_mapped() simply call folio_mapped().  We can now get rid of
folio_large_is_mapped().

_nr_pages_mapped is now only used in rmap code and for debugging purposes.
Keep folio_nr_pages_mapped() around, but document that its use should be
limited to rmap internals and debugging purposes.

This change implies one additional atomic add/sub whenever
mapping/unmapping (parts of) a large folio.

As we now batch RMAP operations for PTE-mapped THP during fork(), during
unmap/zap, and when PTE-remapping a PMD-mapped THP, and we adjust the
large mapcount for a PTE batch only once, the added overhead in the common
case is small.  Only when unmapping individual pages of a large folio
(e.g., during COW), the overhead might be bigger in comparison, but it's
essentially one additional atomic operation.

Note that before the new mapcount would overflow, already our refcount
would overflow: each mapping requires a folio reference.  Extend the
focumentation of folio_mapcount().

Link: https://lkml.kernel.org/r/20240409192301.907377-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Rich Felker <dalias@libc.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 20 insertions, 24 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8fa4d78bcc09..059477821625 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1240,16 +1240,26 @@ static inline int page_mapcount(struct page *page)
 	return mapcount;
 }
 
-int folio_total_mapcount(const struct folio *folio);
+static inline int folio_large_mapcount(const struct folio *folio)
+{
+	VM_WARN_ON_FOLIO(!folio_test_large(folio), folio);
+	return atomic_read(&folio->_large_mapcount) + 1;
+}
 
 /**
- * folio_mapcount() - Calculate the number of mappings of this folio.
+ * folio_mapcount() - Number of mappings of this folio.
  * @folio: The folio.
  *
- * A large folio tracks both how many times the entire folio is mapped,
- * and how many times each individual page in the folio is mapped.
- * This function calculates the total number of times the folio is
- * mapped.
+ * The folio mapcount corresponds to the number of present user page table
+ * entries that reference any part of a folio. Each such present user page
+ * table entry must be paired with exactly on folio reference.
+ *
+ * For ordindary folios, each user page table entry (PTE/PMD/PUD/...) counts
+ * exactly once.
+ *
+ * For hugetlb folios, each abstracted "hugetlb" user page table entry that
+ * references the entire folio counts exactly once, even when such special
+ * page table entries are comprised of multiple ordinary page table entries.
  *
  * Return: The number of times this folio is mapped.
  */
@@ -1257,17 +1267,7 @@ static inline int folio_mapcount(const struct folio *folio)
 {
 	if (likely(!folio_test_large(folio)))
 		return atomic_read(&folio->_mapcount) + 1;
-	return folio_total_mapcount(folio);
-}
-
-static inline bool folio_large_is_mapped(const struct folio *folio)
-{
-	/*
-	 * Reading _entire_mapcount below could be omitted if hugetlb
-	 * participated in incrementing nr_pages_mapped when compound mapped.
-	 */
-	return atomic_read(&folio->_nr_pages_mapped) > 0 ||
-		atomic_read(&folio->_entire_mapcount) >= 0;
+	return folio_large_mapcount(folio);
 }
 
 /**
@@ -1276,11 +1276,9 @@ static inline bool folio_large_is_mapped(const struct folio *folio)
  *
  * Return: True if any page in this folio is referenced by user page tables.
  */
-static inline bool folio_mapped(struct folio *folio)
+static inline bool folio_mapped(const struct folio *folio)
 {
-	if (likely(!folio_test_large(folio)))
-		return atomic_read(&folio->_mapcount) >= 0;
-	return folio_large_is_mapped(folio);
+	return folio_mapcount(folio) >= 1;
 }
 
 /*
@@ -1290,9 +1288,7 @@ static inline bool folio_mapped(struct folio *folio)
  */
 static inline bool page_mapped(const struct page *page)
 {
-	if (likely(!PageCompound(page)))
-		return atomic_read(&page->_mapcount) >= 0;
-	return folio_large_is_mapped(page_folio(page));
+	return folio_mapped(page_folio(page));
 }
 
 static inline struct page *virt_to_head_page(const void *x)