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

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Date:   Fri, 14 May 2021 17:32:15 +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:
> +enum maple_type {
> +	maple_dense,
> +	maple_leaf_64,
> +	maple_range_64,
> +	maple_arange_64,
> +};

> +static inline unsigned long *ma_pivots(struct maple_node *node,
> +					   enum maple_type type)
> +{
> +	switch (type) {
> +	case maple_arange_64:
> +		return node->ma64.pivot;
> +	case maple_range_64:
> +	case maple_leaf_64:
> +		return node->mr64.pivot;
> +	case maple_dense:
> +	default:
> +		return NULL;
> +	}
> +}

> +static inline unsigned long *ma_gaps(struct maple_node *node,
> +				     enum maple_type type)
> +{
> +	switch (type) {
> +	case maple_arange_64:
> +		return node->ma64.gap;
> +	case maple_range_64:
> +	case maple_leaf_64:
> +	case maple_dense:
> +	default:
> +		return NULL;
> +	}
> +}

> +static inline unsigned long mte_pivot(const struct maple_enode *mn,
> +				 unsigned char piv)
> +{
> +	struct maple_node *node = mte_to_node(mn);
> +
> +	switch (mte_node_type(mn)) {
> +	case maple_arange_64:
> +		return node->ma64.pivot[piv];
> +	case maple_range_64:
> +	case maple_leaf_64:
> +		return node->mr64.pivot[piv];
> +	case maple_dense:
> +	default:
> +		return 0;
> +	}
> +}

I would suggest removing the default: case. Without it the cases are
complete and the compiler should not complain. Then if you extend the
enum and forget to add a case, the switch is no longer complete and will
trigger a warning, forcing you to update it (hopefully correct).

If you have the default, you'll not get a warning and it'll do whatever.