On 6/10/2025 7:56 AM, Alistair Popple wrote:

On Mon, Jun 09, 2025 at 05:08:07PM +0000, Shivank Garg wrote:

quoted

Replace open-coded folio reference count calculations with the
folio_expected_ref_count() helper to improve code maintainability
and reduce duplication.

I wonder if there is any opportunity for reducing duplication more broadly?
The migration code has similar helpers (folio_expected_refs) as does
khugepaged (is_refcount_suitable) and vmscan (is_page_cache_freeable).

The folio_expected_refs() and is_refcount_suitable() consolidation was 
recently merged:
- 86ebd50224c0 ("mm: add folio_expected_ref_count() for reference count calculation")
- 0b43b8bc8ef8 ("mm/khugepaged: clean up refcount check using folio_expected_ref_count()")

do_huge_pmd_wp_page() also has an open-coded version of these checks and there
are probably others around the place to.

These could all be converted to a helper that returns all the "extra" references
after taking into account things like mapping, swapcache, etc. depending on folio.> 

quoted

No functional changes intended.

Signed-off-by: Shivank Garg <redacted>
---
 kernel/events/uprobes.c | 5 +++--
 mm/memfd.c              | 4 ++--
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 4c965ba77f9f..c978c8c27340 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c

@@ -434,10 +434,11 @@ static int __uprobe_write_opcode(struct vm_area_struct *vma,
 	/*
 	 * When unregistering, we may only zap a PTE if uffd is disabled and
 	 * there are no unexpected folio references ...
+	 * Expected refs: mappings + swapcache.
+	 * We hold one additional reference (+1).
 	 */
 	if (is_register || userfaultfd_missing(vma) ||
-	    (folio_ref_count(folio) != folio_mapcount(folio) + 1 +
-	     folio_test_swapcache(folio) * folio_nr_pages(folio)))
+	    (folio_ref_count(folio) != folio_expected_ref_count(folio) + 1))
 		goto remap;
 
 	/*

diff --git a/mm/memfd.c b/mm/memfd.c
index ab367e61553d..4ed5506221b7 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c

@@ -32,8 +32,8 @@
 
 static bool memfd_folio_has_extra_refs(struct folio *folio)
 {
-	return folio_ref_count(folio) - folio_mapcount(folio) !=
-	       folio_nr_pages(folio);
+	/* Expected refs: pagecache + mappings */
+	return folio_ref_count(folio) != folio_expected_ref_count(folio);
 }
 
 static void memfd_tag_pins(struct xa_state *xas)

-- 
2.43.0