[RFC PATCH 3/5] mm/mlock: WARN_ON() if mapcount overflows into mlock_count

[Yosry Ahmed <yosryahmed@xxxxxxxxxx>]

Add a hook when a new mapping is added to an order-0 mlocked folio to
check if the mapcount overflowed beyond the allocated 20 bits into the
mlock_count. This is useful to know if this alarm if this happens
frequently enough to cause a problem.

We do so by checking if the folio has the lower 20 bits as all 0s. For
file-backed folios, we do not hold the folio lock while adding a new
mapping, so there's a chance that two mappings are added in quick
succession such that the warning doesn't fire. Don't sweat it.

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 include/linux/mm.h |  4 ++++
 mm/mlock.c         | 13 +++++++++++++
 mm/rmap.c          |  2 ++
 3 files changed, 19 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b341477a83e8..917f81996e22 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1050,6 +1050,7 @@ unsigned long vmalloc_to_pfn(const void *addr);
 extern bool is_vmalloc_addr(const void *x);
 extern int is_vmalloc_or_module_addr(const void *x);
 extern int folio_mlocked_mapcount(struct folio *folio);
+extern void folio_mlock_map_check(struct folio *folio);
 extern void folio_mlock_unmap_check(struct folio *folio);
 #else
 static inline bool is_vmalloc_addr(const void *x)
@@ -1064,6 +1065,9 @@ static inline int folio_mlocked_mapcount(struct folio *folio)
 {
 	return 0;
 }
+static inline void folio_mlock_map_check(struct folio *folio)
+{
+}
 static inline void folio_mlock_unmap_check(struct folio *folio)
 {
 }
diff --git a/mm/mlock.c b/mm/mlock.c
index 8261df11d6a6..f8b3fb1b2986 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -140,6 +140,19 @@ static int folio_mlock_count_dec(struct folio *folio)
 	return mlock_count - 1;
 }
 
+/*
+ * Call after incrementing the mapcount. WARN_ON() if the mapcount overflows
+ * beyond the lower 20 bits for order-0 mlocked folios.
+ */
+void folio_mlock_map_check(struct folio *folio)
+{
+	int mapcount = atomic_read(&folio->_mapcount) + 1;
+
+	/* WARN if we overflow beyond the lower 20 bits */
+	if (unlikely(!folio_test_large(folio) && folio_test_mlocked(folio)))
+		WARN_ON((mapcount & MLOCK_MAPCOUNT_MASK) == 0);
+}
+
 /*
  * Call after decrementing the mapcount. If the mapcount previously overflowed
  * beyond the lower 20 bits for an order-0 mlocked folio, munlock() have
diff --git a/mm/rmap.c b/mm/rmap.c
index 02e558551f15..092529319782 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1228,6 +1228,7 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 			nr = atomic_inc_return_relaxed(mapped);
 			nr = (nr < COMPOUND_MAPPED);
 		}
+		folio_mlock_map_check(folio);
 	} else if (folio_test_pmd_mappable(folio)) {
 		/* That test is redundant: it's for safety or to optimize out */
 
@@ -1330,6 +1331,7 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
 			nr = atomic_inc_return_relaxed(mapped);
 			nr = (nr < COMPOUND_MAPPED);
 		}
+		folio_mlock_map_check(folio);
 	} else if (folio_test_pmd_mappable(folio)) {
 		/* That test is redundant: it's for safety or to optimize out */
 
-- 
2.41.0.162.gfafddb0af9-goog