linux-kernel - Re: [PATCH v2 04/15] lib min_heap: Add type safe interface

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Date: Sat, 23 Mar 2024 01:02:29 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: colyli@...e.de, msakai@...hat.com, peterz@...radead.org,
	mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
	akpm@...ux-foundation.org, bfoster@...hat.com, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com, jserv@...s.ncku.edu.tw,
	dm-devel@...ts.linux.dev, linux-bcache@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-bcachefs@...r.kernel.org,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 04/15] lib min_heap: Add type safe interface

On Thu, Mar 21, 2024 at 05:22:14PM -0400, Kent Overstreet wrote:
> On Thu, Mar 21, 2024 at 07:57:47PM +0800, Kuan-Wei Chiu wrote:
> > On Wed, Mar 20, 2024 at 04:56:57PM -0400, Kent Overstreet wrote:
> > > On Wed, Mar 20, 2024 at 10:54:06PM +0800, Kuan-Wei Chiu wrote:
> > > > Introduce a type-safe interface for min_heap by adding small macro
> > > > wrappers around functions and using a 0-size array to store type
> > > > information. This enables the use of __minheap_cast and
> > > > __minheap_obj_size macros for type casting and obtaining element size.
> > > > The implementation draws inspiration from generic-radix-tree.h,
> > > > eliminating the need to pass element size in min_heap_callbacks.
> > > 
> > > let's avoid the heap->heap.nr - darray (fs/bcachefs/darray.h) has a
> > > trick for that. All heaps have the same memory layout, so we can just
> > > cast to a void pointer heap to get something the C code can use.
> > >
> > If I understand correctly, you're suggesting adding APIs similar to
> > darray_top(), darray_first(), and darray_last() within min_heap and
> > having them return a pointer. However, some users are using heap.nr in
> > conditional statements instead of utilizing heap.nr for memory
> > operations, so returning pointers may not be as convenient. What about
> > adding get and set functions for nr instead?
> 
> No, I mean not having separate inner and outer types. Want me to sketch
> something out?

Based on your suggestion, I've come up with the following code snippet:

#define MIN_HEAP_PREALLOCATED(_type, _name, _nr) \
struct _name {  \
    int nr; \
    int size;   \
    _type *data;    \
    _type preallocated[_nr];    \
};

#define MIN_HEAP(_type, _name) MIN_HEAP_PREALLOCATED(_type, _name, 0)

typdef MIN_HEAP(char, _) min_heap_char;

static __always_inline
void min_heap_init(min_heap_char *heap, void *data, int size)
{
	heap->nr = 0;
	heap->size = size;
    heap->data = size <= ARRAY_SIZE(heap->preallocated) ? heap->preallocated : data;
}

But I'm not sure how to implement other inline functions like
min_heap_push or min_heap_pop if I do that, unless they are rewritten
using macros. Also, I'm not sure how to make the less and swp functions
in the min_heap_callbacks not use void * type parameters. Or perhaps I
misunderstood your meaning again. If you could sketch out your idea or
have a better approach, it would be a great help to me. Any guidance
would be greatly appreciated.

Regards,
Kuan-Wei