Re: [RFC PATCH 3/3] fb_defio: do not use deprecated page->mapping, index fields

[Matthew Wilcox]

On Wed, Jan 08, 2025 at 04:18:42PM +0000, Lorenzo Stoakes wrote:
> @@ -280,7 +269,10 @@ static void fb_deferred_io_work(struct work_struct *work)
>  		struct folio *folio = page_folio(pageref->page);
>  
>  		folio_lock(folio);
> -		folio_mkclean(folio);
> +		rmap_wrprotect_file_page(fbdefio->mapping,
> +					 pageref->offset >> PAGE_SHIFT,
> +					 compound_nr(pageref->page),
> +					 page_to_pfn(pageref->page));
>  		folio_unlock(folio);

Why do we need to lock the folio?  (since this isn't necessarily a
folio)  Also, do we need compound_nr() here?  I _think_ for defio,
the number of pages allocated per object are fixed, so this should be
an fbdefio->nr_pages field?

(something that's always troubled me about compound_nr() is that it
returns 1 for tail pages and the number you actually expect for head
pages)