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

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Message-ID: <398d49f6-00c8-4bd8-ba30-62cbf0efee46@redhat.com>
Date: Tue, 10 Jun 2025 09:08:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Shivank Garg <shivankg@....com>, 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, akpm@...ux-foundation.org
Cc: 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 09.06.25 19:08, Shivank Garg wrote:
> Replace open-coded folio reference count calculations with the
> folio_expected_ref_count() helper to improve code maintainability
> and reduce duplication.
> 
> 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;
>   

With the comment removed (the caller holds a reference from GUP) or 
simplified ("Caller holds an additional folio reference.")

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb