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

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Message-ID: <YJ5bbw70JLfNi8Q7@hirez.programming.kicks-ass.net>
Date:   Fri, 14 May 2021 13:13:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "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:

> +/*
> + * Special values for ma_state.node.
> + * MAS_START means we have not searched the tree.
> + * MAS_ROOT means we have searched the tree and the entry we found lives in
> + * the root of the tree (ie it has index 0, length 1 and is the only entry in
> + * the tree).
> + * MAS_NONE means we have searched the tree and there is no node in the
> + * tree for this entry.  For example, we searched for index 1 in an empty
> + * tree.  Or we have a tree which points to a full leaf node and we
> + * searched for an entry which is larger than can be contained in that
> + * leaf node.
> + * MA_ERROR represents an errno.  After dropping the lock and attempting
> + * to resolve the error, the walk would have to be restarted from the
> + * top of the tree as the tree may have been modified.
> + */
> +#define MAS_START	((struct maple_enode *)1UL)
> +#define MAS_ROOT	((struct maple_enode *)5UL)
> +#define MAS_NONE	((struct maple_enode *)9UL)
> +#define MA_ERROR(err) \
> +		((struct maple_enode *)(((unsigned long)err << 2) | 2UL))
> +

> +static inline enum maple_type mte_node_type(const struct maple_enode *entry)
> +{
> +	return ((unsigned long)entry >> 3) & 15;
> +}

> +static inline struct maple_node *mte_to_node(const struct maple_enode *entry)
> +{
> +	return (struct maple_node *)((unsigned long)entry & ~127);
> +}

> +static inline struct maple_topiary *mte_to_mat(const struct maple_enode *entry)
> +{
> +	return (struct maple_topiary *)((unsigned long)entry & ~127);
> +}

Can we please write masks as hex, also do they want a pretty name?


This has more magic mask values, proper names might be good:

> +static inline void mte_set_parent(struct maple_enode *enode,
> +				 const struct maple_enode *parent,
> +				 unsigned char slot)
> +{
> +	unsigned long bitmask = 0x78;
> +	unsigned long val = (unsigned long) parent;
> +	unsigned long type = 0;
> +
> +	switch (mte_node_type(parent)) {
> +	case maple_range_64:
> +	case maple_arange_64:
> +		type = 6;
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	val &= ~bitmask; // Remove any old slot number.
> +	val |= (slot << MAPLE_PARENT_SHIFT); // Set the slot.
> +	val |= type;
> +	mte_to_node(enode)->parent = ma_parent_ptr(val);
> +}

> +static inline unsigned int mte_parent_slot(const struct maple_enode *enode)
> +{
> +	unsigned long bitmask = 0x7C;
> +	unsigned long val = (unsigned long) mte_to_node(enode)->parent;
> +
> +	if (val & 1)
> +		return 0; // Root.
> +
> +	return (val & bitmask) >> MAPLE_PARENT_SHIFT;

7c is 1111100, but then you're shifting out the one bit that makes the
difference from the above magic 0x78. What gives?

> +}