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xarray: Replace exceptional entries

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Introduce xarray value entries and tagged pointers to replace radix
tree exceptional entries.  This is a slight change in encoding to allow
the use of an extra bit (we can now store BITS_PER_LONG - 1 bits in a
value entry).  It is also a change in emphasis; exceptional entries are
intimidating and different.  As the comment explains, you can choose
to store values or pointers in the xarray and they are both first-class
citizens.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>
This commit is contained in:
Matthew Wilcox
2017-11-03 13:30:42 -04:00
parent 66ee620f06
commit 3159f943aa
26 changed files with 278 additions and 232 deletions
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36
include/linux/radix-tree.h
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@@ -28,34 +28,26 @@
#include <linux/rcupdate.h>
#include <linux/spinlock.h>
#include <linux/types.h>
#include <linux/xarray.h>
/*
* The bottom two bits of the slot determine how the remaining bits in the
* slot are interpreted:
*
* 00 - data pointer
* 01 - internal entry
* 10 - exceptional entry
* 11 - this bit combination is currently unused/reserved
* 10 - internal entry
* x1 - value entry
*
* The internal entry may be a pointer to the next level in the tree, a
* sibling entry, or an indicator that the entry in this slot has been moved
* to another location in the tree and the lookup should be restarted. While
* NULL fits the 'data pointer' pattern, it means that there is no entry in
* the tree for this index (no matter what level of the tree it is found at).
* This means that you cannot store NULL in the tree as a value for the index.
* This means that storing a NULL entry in the tree is the same as deleting
* the entry from the tree.
*/
#define RADIX_TREE_ENTRY_MASK 3UL
#define RADIX_TREE_INTERNAL_NODE 1UL
/*
* Most users of the radix tree store pointers but shmem/tmpfs stores swap
* entries in the same tree. They are marked as exceptional entries to
* distinguish them from pointers to struct page.
* EXCEPTIONAL_ENTRY tests the bit, EXCEPTIONAL_SHIFT shifts content past it.
*/
#define RADIX_TREE_EXCEPTIONAL_ENTRY 2
#define RADIX_TREE_EXCEPTIONAL_SHIFT 2
#define RADIX_TREE_INTERNAL_NODE 2UL
static inline bool radix_tree_is_internal_node(void *ptr)
{
@@ -83,11 +75,10 @@ static inline bool radix_tree_is_internal_node(void *ptr)
/*
* @count is the count of every non-NULL element in the ->slots array
* whether that is an exceptional entry, a retry entry, a user pointer,
* whether that is a value entry, a retry entry, a user pointer,
* a sibling entry or a pointer to the next level of the tree.
* @exceptional is the count of every element in ->slots which is
* either radix_tree_exceptional_entry() or is a sibling entry for an
* exceptional entry.
* either a value entry or a sibling of a value entry.
*/
struct radix_tree_node {
unsigned char shift; /* Bits remaining in each slot */
@@ -268,17 +259,6 @@ static inline int radix_tree_deref_retry(void *arg)
return unlikely(radix_tree_is_internal_node(arg));
}
/**
* radix_tree_exceptional_entry - radix_tree_deref_slot gave exceptional entry?
* @arg: value returned by radix_tree_deref_slot
* Returns: 0 if well-aligned pointer, non-0 if exceptional entry.
*/
static inline int radix_tree_exceptional_entry(void *arg)
{
/* Not unlikely because radix_tree_exception often tested first */
return (unsigned long)arg & RADIX_TREE_EXCEPTIONAL_ENTRY;
}
/**
* radix_tree_exception - radix_tree_deref_slot returned either exception?
* @arg: value returned by radix_tree_deref_slot