Re: [PATCH v1 03/11] mm: simplify hugetlb and file-THP handling in __page_mapcount()

[Nadav Amit]

> On Dec 17, 2021, at 3:30 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
> Let's return early for hugetlb, which really only relies on the compound
> mapcount so far and does not support PageDoubleMap() yet. Use the chance
> to cleanup the file-THP case to make it easier to grasp. While at it, use
> head_compound_mapcount().
> 
> This is a preparation for further changes.

It would be useful to add “no functional change intended” or something.

> 
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> mm/util.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 741ba32a43ac..3239e75c148d 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -732,15 +732,18 @@ int __page_mapcount(struct page *page)
> {
> 	int ret;
> 
> -	ret = atomic_read(&page->_mapcount) + 1;
> +	if (PageHuge(page))
> +		return compound_mapcount(page);

Before you return, perhaps you can add an assertion like:

	VM_BUG_ON(PageDoubleMap(page));

This would be make the code clearer and would ease debugging in the
future (if support for double-map is expanded).