Re: [PATCH v3 09/14] mm/page-flags: check page/folio->private instead of PG_private

On Tue Sep 8, 2026 at 12:56 PM EDT, David Hildenbrand (Arm) wrote:

On 9/8/26 04:56, Zi Yan wrote:

quoted

After the changes of the prior commits, page/folio->private != NULL is now
equivalent to checking PG_private.

Stop checking PG_private on pages and folios and use page/folio->private
instead, except swapcache and hugetlb folios, because the former uses a
field (swp_entry_t swap) overlapping with ->private and the latter sets its
flags in ->private. Exclude swapcache and hugetlb when the code is meant to
check PG_private only. PG_swapcache and folio->swap.val cannot be set/clear
as a whole, so excluding swapcache with folio_test_swapcache() is not
reliable. Instead, use folio_test_swapbacked(), since PG_swapbacked is
stable when a folio is added to/removed from swapcache.

folio_expected_ref_count() can be called without folio lock, so annotate
folio_test_private() with data_race() to avoid triggering race condition
checks. While at it, annotate folio->mapping too. Add data_race()
annotation for other lockless callers too.

folio_set/clear_private() and Set/ClearPagePrivate() become no-ops.
PG_private is no longer checked at page free time.

Remove KPF_PRIVATE since PG_private is no longer used.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: David Hildenbrand <david@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
To: Lorenzo Stoakes <ljs@kernel.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Jan Kara <jack@suse.cz>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Kairui Song <kasong@tencent.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-trace-kernel@vger.kernel.org
---

[...]

quoted

 		/* !PageAnon && !swapper_space */

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 40d3f1b48a74c..9348ebf9de882 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -978,7 +978,8 @@ static void folio_check_dirty_writeback(struct folio *folio,
 	*writeback = folio_test_writeback(folio);
 
 	/* Verify dirty/writeback state if the filesystem supports it */
-	if (!folio_test_private(folio))
+	if (!(folio_test_private(folio) && !folio_test_swapbacked(folio) &&
+	      !folio_test_hugetlb(folio)))

Now that I read that check ... a fourth time? a fifth? I stopped counting :)

Should we have a (in light of folio_attach_private()):

static inline bool folio_has_attached_private(const struct folio *folio)
{
	/* Careful, we might get called on unlocked folios. */
	if (!data_race(folio->private))
		return false;
	/* On some folios ->private is used for different purposes. */
	return !folio_test_swapbacked(folio) && !folio_test_hugetlb(folio);
}

Of course, adding some nice documentation what having attached private means?

Will squash the next one into this one and use
folio_has_attached_private() instead of folio_test_fs_private(), like we
discussed in Patch 10.



-- 
Best Regards,
Yan, Zi