Re: [PATCH v1 06/18] mm: make folio_mapcount() return 0 for small typed folios

[David Hildenbrand]

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 <da...@redhat.com>
---
  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 <da...@redhat.com>
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 <da...@redhat.com>
---
 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