Re: [PATCH 4/7] brd: make sector size configurable

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 14, 2023 at 01:46:34PM +0200, Hannes Reinecke wrote:
> @@ -43,9 +43,11 @@ struct brd_device {
>  	 */
>  	struct xarray	        brd_folios;
>  	u64			brd_nr_folios;
> +	unsigned int		brd_sector_shift;
> +	unsigned int		brd_sector_size;
>  };
>  
> -#define BRD_SECTOR_SHIFT(b) (PAGE_SHIFT - SECTOR_SHIFT)
> +#define BRD_SECTOR_SHIFT(b) ((b)->brd_sector_shift - SECTOR_SHIFT)
>  
>  static pgoff_t brd_sector_index(struct brd_device *brd, sector_t sector)
>  {
> @@ -85,7 +87,7 @@ static int brd_insert_folio(struct brd_device *brd, sector_t sector, gfp_t gfp)
>  {
>  	pgoff_t idx;
>  	struct folio *folio, *cur;
> -	unsigned int rd_sector_order = get_order(PAGE_SIZE);
> +	unsigned int rd_sector_order = get_order(brd->brd_sector_size);

Surely max(0, brd->brd_sector_shift - PAGE_SHIFT) ?

> @@ -346,6 +353,25 @@ static int brd_alloc(int i)
>  		return -ENOMEM;
>  	brd->brd_number		= i;
>  	list_add_tail(&brd->brd_list, &brd_devices);
> +	brd->brd_sector_shift = ilog2(rd_blksize);
> +	if ((1ULL << brd->brd_sector_shift) != rd_blksize) {
> +		pr_err("rd_blksize %d is not supported\n", rd_blksize);

Are you trying to require power-of-two here?  We have is_power_of_2()
for that purpose.