Re: [RESEND PATCH v5 04/16] lib min_heap: Add type safe interface

Andrew Morton
2024-05-22T23:10:49Z

On Tue, 14 May 2024 16:47:12 +0800 Kuan-Wei Chiu <visitorckw@gmail.com> wrote:

> Implement a type-safe interface for min_heap using strong type
> pointers instead of void * in the data field. This change includes
> adding small macro wrappers around functions, enabling the use of
> __minheap_cast and __minheap_obj_size macros for type casting and
> obtaining element size. This implementation removes the necessity of
> passing element size in min_heap_callbacks. Additionally, introduce the
> MIN_HEAP_PREALLOCATED macro for preallocating some elements.

I guess it's a nice change, although it's unclear how valuable all this
is. Being able to remove the local implementations in bcache and
bcachefs is good.

> --- a/drivers/md/dm-vdo/repair.c
> +++ b/drivers/md/dm-vdo/repair.c
> @@ -51,6 +51,8 @@ struct recovery_point {
> bool increment_applied;
> };
>
> +MIN_HEAP(struct numbered_block_mapping, replay_heap);
>
> ...
>
> -struct min_heap {
> - void *data;
> - int nr;
> - int size;
> -};
> +#define MIN_HEAP_PREALLOCATED(_type, _name, _nr) \
> +struct _name { \
> + int nr; \
> + int size; \
> + _type *data; \
> + _type preallocated[_nr]; \
> +}

I think that the MIN_HEAP() macro would be better named
DEFINE_MIN_HEAP(). There's a lot of precedent for this and it's
unclear whether "MIN_HEAP" actually implements storage. I looked at
the repair.c addition and thought "hm, shouldn't that be static"!

> /* Sift the element at pos down the heap. */
> static __always_inline
> -void min_heapify(struct min_heap *heap, int pos,
> +void __min_heapify(min_heap_char *heap, int pos, size_t elem_size,
> const struct min_heap_callbacks *func)
> {

It's a separate project, but min_heap.h has some crazily huge inlined
functions. I expect we'd have a smaller and faster kernel if those
were moved into a new lib/min_heap.c.