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

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Date:   Fri, 14 May 2021 17:40:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...riel.com>,
        Michel Lespinasse <walken.cr@...il.com>
Subject: Re: [PATCH 26/94] Maple Tree: Add new data structure

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.