Re: [PATCH 06/12] mm/truncate: add folio_unmap_invalidate() helper

[Matthew Wilcox <willy@infradead.org>]

On Fri, Dec 20, 2024 at 08:47:44AM -0700, Jens Axboe wrote:
> +int folio_unmap_invalidate(struct address_space *mapping, struct folio *folio,
> +			   gfp_t gfp)
>  {
> -	if (folio->mapping != mapping)
> -		return 0;
> +	int ret;
> +
> +	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>  
> -	if (!filemap_release_folio(folio, GFP_KERNEL))
> +	if (folio_test_dirty(folio))
>  		return 0;
> +	if (folio_mapped(folio))
> +		unmap_mapping_folio(folio);
> +	BUG_ON(folio_mapped(folio));
> +
> +	ret = folio_launder(mapping, folio);
> +	if (ret)
> +		return ret;
> +	if (folio->mapping != mapping)
> +		return -EBUSY;

The position of this test confuses me.  Usually we want to test
folio->mapping early on, since if the folio is no longer part of this
file, we want to stop doing things to it, rather than go to the trouble
of unmapping it.  Also, why do we want to return -EBUSY in this case?
If the folio is no longer part of this file, it has been successfully
removed from this file, right?