linux-kernel - Re: [RESEND PATCH v5 04/16] lib min

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Date: Wed, 22 May 2024 16:10:48 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: colyli@...e.de, kent.overstreet@...ux.dev, msakai@...hat.com,
 peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
 namhyung@...nel.org, bfoster@...hat.com, mark.rutland@....com,
 alexander.shishkin@...ux.intel.com, jolsa@...nel.org, irogers@...gle.com,
 adrian.hunter@...el.com, bagasdotme@...il.com, jserv@...s.ncku.edu.tw,
 linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
 dm-devel@...ts.linux.dev, linux-bcachefs@...r.kernel.org,
 linux-perf-users@...r.kernel.org
Subject: Re: [RESEND PATCH v5 04/16] lib min_heap: Add type safe interface

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