Re: [PATCH 03/19] jbd2: point the shadow buffer at the frozen data directly

[Matthew Wilcox]

On Sat, Aug 01, 2026 at 06:00:47PM -0400, Chao Shi wrote:
> @@ -330,6 +330,8 @@ static __u32 jbd2_checksum_data(__u32 crc32_sum, struct buffer_head *bh)
>  	char *addr;
>  	__u32 checksum;
>  
> +	if (!bh->b_folio)
> +		return crc32_be(crc32_sum, bh->b_data, bh->b_size);
>  	addr = kmap_local_folio(bh->b_folio, bh_offset(bh));
>  	checksum = crc32_be(crc32_sum, addr, bh->b_size);
>  	kunmap_local(addr);

This is awkward.  How about ...

static inline void *kmap_local_bh(const struct buffer_head *bh)
{
	if (bh->b_folio)
		return kmap_local_folio(bh->b_folio, bh_offset(bh));
	return kmap_local_page(virt_to_page(bh->b_data);
}

(this is also somewhat awkward because it feels like we could just
return bh->b_data, but kunmap_local_indexed() does some ... stuff)

Anyway, it all gets optimised away on non-HIGHMEM.  Or if it doesn't,
you can force it to ;-)