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

[David Hildenbrand (Arm)]

On 9/8/26 04:56, Zi Yan wrote:
> 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@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: David Hildenbrand <david@xxxxxxxxxx>
> To: Steven Rostedt <rostedt@xxxxxxxxxxx>
> To: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> To: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> To: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> To: Jan Kara <jack@xxxxxxx>
> To: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: "Liam R. Howlett" <liam@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Nico Pache <nico.pache@xxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Dev Jain <dev.jain@xxxxxxx>
> Cc: Barry Song <baohua@xxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Usama Arif <usama.arif@xxxxxxxxx>
> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> Cc: Rakie Kim <rakie.kim@xxxxxx>
> Cc: Byungchul Park <byungchul@xxxxxx>
> Cc: Gregory Price <gourry@xxxxxxxxxx>
> Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Qi Zheng <qi.zheng@xxxxxxxxx>
> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> Cc: Yuanchu Xie <yuanchu@xxxxxxxxxx>
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
> ---

[...]

>  		/* !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?

-- 
Cheers,

David