Re: [PATCH v3 0/5] Keep tail page private zero at free and folio split time

[Andrew Morton]

On Thu, 09 Jul 2026 14:06:00 -0400 Zi Yan <ziy@xxxxxxxxxx> wrote:

> Hi all,
> 
> This patchset makes sure tail_page->private is zero before compound or
> high-order pages are returned to the allocator. It also checks tail pages
> that become new folio heads during large folio split, before their private
> fields are used by new folios.

Updated, thanks.

> Changes in v3:
> 1. replaced "it" with "a DEBUG_VM check" in Patch 3 commit message
> 2. moved tail_page->private bad_page() code up to the existing
>    is_check_pages_enabled() block in Patch 4.
> - Link to v2: https://lore.kernel.org/r/20260703-keep-subpage-private-zero-at-free-v2-0-2970fe777dd6@xxxxxxxxxx

Here's how v3 altered mm.git:


 mm/page_alloc.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/mm/page_alloc.c~b
+++ a/mm/page_alloc.c
@@ -1386,13 +1386,14 @@ static __always_inline bool __free_pages
 					bad++;
 					continue;
 				}
+
+				if (tail_page->private) {
+					bad_page(tail_page, "nonzero private");
+					bad++;
+					continue;
+				}
 			}
 			tail_page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
-			if (is_check_pages_enabled() && tail_page->private) {
-				bad_page(tail_page, "nonzero private");
-				bad++;
-				continue;
-			}
 		}
 	}
 	if (folio_test_anon(folio)) {
_