[RFC,3/5] mm/mlock: WARN_ON() if mapcount overflows into mlock_count
Commit Message
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@google.com>
---
include/linux/mm.h | 4 ++++
mm/mlock.c | 13 +++++++++++++
mm/rmap.c | 2 ++
3 files changed, 19 insertions(+)
@@ -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)
{
}
@@ -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
@@ -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 */