linux-kernel - Re: [RFC] mm: use folio_expected_ref_count() helper for reference counting

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Message-ID: <yhmscjk4j22mi6yjqpswighh2h5phnmdc7cr2otvn43ng4w577@vheecvjlwmmr>
Date: Tue, 10 Jun 2025 12:26:44 +1000
From: Alistair Popple <apopple@...dia.com>
To: Shivank Garg <shivankg@....com>
Cc: mhiramat@...nel.org, oleg@...hat.com, peterz@...radead.org, 
	mingo@...hat.com, acme@...nel.org, namhyung@...nel.org, mark.rutland@....com, 
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com, 
	kan.liang@...ux.intel.com, david@...hat.com, akpm@...ux-foundation.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC] mm: use folio_expected_ref_count() helper for reference
 counting

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

> No functional changes intended.
> 
> Signed-off-by: Shivank Garg <shivankg@....com>
> ---
>  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
> 
>