On 09.04.24 21:22, David Hildenbrand wrote:
We already handle it properly for large folios. Let's also return "0"
for small typed folios, like page_mapcount() currently would.
Consequently, folio_mapcount() will never return negative values for
typed folios, but may return negative values for underflows.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
include/linux/mm.h | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index daf687f0e8e5..d453232bba62 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1260,12 +1260,19 @@ static inline int folio_large_mapcount(const struct folio *folio)
* references the entire folio counts exactly once, even when such special
* page table entries are comprised of multiple ordinary page table entries.
*
+ * Will report 0 for pages which cannot be mapped into userspace, such as
+ * slab, page tables and similar.
+ *
* Return: The number of times this folio is mapped.
*/
static inline int folio_mapcount(const struct folio *folio)
{
- if (likely(!folio_test_large(folio)))
- return atomic_read(&folio->_mapcount) + 1;
+ int mapcount;
+
+ if (likely(!folio_test_large(folio))) {
+ mapcount = atomic_read(&folio->_mapcount);
+ return page_type_has_type(mapcount) ? 0 : mapcount + 1;
+ }
return folio_large_mapcount(folio);
}
From 98acfb7ff35cb65fcfca5e799bf58f8afe84a645 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@xxxxxxxxxx>
Date: Wed, 24 Apr 2024 10:56:17 +0200
Subject: [PATCH] !fixup: mm: make folio_mapcount() return 0 for small typed
folios
Just like page_mapcount(), let's make folio_mapcount() slightly more
efficient.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
include/linux/mm.h | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index cf700c5cdd58b..78e583b50e421 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1271,8 +1271,11 @@ static inline int folio_mapcount(const struct folio *folio)
int mapcount;
if (likely(!folio_test_large(folio))) {
- mapcount = atomic_read(&folio->_mapcount);
- return page_type_has_type(mapcount) ? 0 : mapcount + 1;
+ mapcount = atomic_read(&folio->_mapcount) + 1;
+ /* Handle page_has_type() pages */
+ if (mapcount < PAGE_MAPCOUNT_RESERVE + 1)
+ mapcount = 0;
+ return mapcount;
}
return folio_large_mapcount(folio);
}
--
2.44.0
--
Cheers,
David / dhildenb
