[PATCH 2/3] bpf: arena: use page_ref_count() instead of page_mapped() in arena_free_pages()

[David Hildenbrand (Arm)]

Pages that BPF arena code maps are allocated through
bpf_map_alloc_pages(), which does not allocate folios but pages.

In the future, pages will not have a mapcount, only folios will.
Converting the code to use folios and rely on folio_mapped() sounds like
the wrong approach.

Should BPF arena code allocate folios and use folio_mapped() here? But
likely we would not want to use folios here longterm, as we don't really
need folio information.

Hard to tell. But in the meantime, we can simply use the page refcount
instead, as a heuristic whether the page might be mapped to user space
and we would want to try zapping it, so we can get rid of page_mapped().

Page allocation will give us a page with a refcount of 1. Any user space
mapping adds a page reference. While there can be references from other
subsystems (e.g., GUP), in the common case for this test here relying on
the page count is good enough.

Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
 kernel/bpf/arena.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 802656c6fd3c..608c55c260bc 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -729,7 +729,7 @@ static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt,
 
 	llist_for_each_safe(pos, t, __llist_del_all(&free_pages)) {
 		page = llist_entry(pos, struct page, pcp_llist);
-		if (page_cnt == 1 && page_mapped(page)) /* mapped by some user process */
+		if (page_cnt == 1 && page_ref_count(page) > 1) /* maybe mapped by user space */
 			/* Optimization for the common case of page_cnt==1:
 			 * If page wasn't mapped into some user vma there
 			 * is no need to call zap_pages which is slow. When

-- 
2.43.0