Re: [PATCH 26/94] Maple Tree: Add new data structure

[Peter Zijlstra]

On Wed, Apr 28, 2021 at 03:36:02PM +0000, Liam Howlett wrote:
> +/*
> + * mas_set_alloc_req() - Set the requested number of allocations.
> + * @mas: the maple state
> + * @count: the number of allocations.
> + *
> + * If @mas->alloc has bit 1 set (0x1) or @mas->alloc is %NULL, then there are no
> + * nodes allocated and @mas->alloc should be set to count << 1 | 1.  If there is
> + * already nodes allocated, then @mas->alloc->request_count stores the request.
> + */
> +static inline void mas_set_alloc_req(struct ma_state *mas, unsigned long count)
> +{
> +	if (!mas->alloc || ((unsigned long)mas->alloc & 0x1)) {
> +		if (!count)
> +			mas->alloc = NULL;
> +		else
> +			mas->alloc = (struct maple_alloc *)(((count) << 1U) | 1U);
> +		return;
> +	}
> +
> +	mas->alloc->request_count = count;
> +}
> +
> +/*
> + * mas_alloc_req() - get the requested number of allocations.
> + * @mas: The maple state
> + *
> + * The alloc count is either stored directly in @mas, or in
> + * @mas->alloc->request_count if there is at least one node allocated.
> + *
> + * Return: The allocation request count.
> + */
> +static inline unsigned int mas_alloc_req(const struct ma_state *mas)
> +{
> +	if ((unsigned long)mas->alloc & 0x1)
> +		return (unsigned long)(mas->alloc) >> 1;
> +	else if (mas->alloc)
> +		return mas->alloc->request_count;
> +	return 0;
> +}


I'm confuse.. and the comments fail to eludicate *why* the code is the
way it is, they simply explain exactly what the code does, which I can
already tell from reading the code.

Why can't we have ->request_count unconditionally be the requested
count, and have ->alloc be NULL or not. Why do we play games with low
pointer bits here? AFAICT there's no actual benefit to doing so.