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    Subject[PATCH v4 05/73] xarray: Change definition of sibling entries
    Date
    From: Matthew Wilcox <mawilcox@microsoft.com>

    Instead of storing a pointer to the slot containing the canonical entry,
    store the offset of the slot. Produces slightly more efficient code
    (~300 bytes) and simplifies the implementation.

    Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
    ---
    include/linux/xarray.h | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++
    lib/radix-tree.c | 65 +++++++++++----------------------------
    2 files changed, 100 insertions(+), 47 deletions(-)

    diff --git a/include/linux/xarray.h b/include/linux/xarray.h
    index e55f5cfd14ed..2c45d87a3476 100644
    --- a/include/linux/xarray.h
    +++ b/include/linux/xarray.h
    @@ -58,6 +58,8 @@ static inline bool xa_is_value(void *entry)
    return (unsigned long)entry & 1;
    }

    +/* Everything below here is the Advanced API. Proceed with caution. */
    +
    #define xa_trylock(xa) spin_trylock(&(xa)->xa_lock)
    #define xa_lock(xa) spin_lock(&(xa)->xa_lock)
    #define xa_unlock(xa) spin_unlock(&(xa)->xa_lock)
    @@ -71,4 +73,84 @@ static inline bool xa_is_value(void *entry)
    spin_unlock_irqrestore(&(xa)->xa_lock, flags)
    #define xa_lock_held(xa) lockdep_is_held(&(xa)->xa_lock)

    +/*
    + * The xarray is constructed out of a set of 'chunks' of pointers. Choosing
    + * the best chunk size requires some tradeoffs. A power of two recommends
    + * itself so that we can walk the tree based purely on shifts and masks.
    + * Generally, the larger the better; as the number of slots per level of the
    + * tree increases, the less tall the tree needs to be. But that needs to be
    + * balanced against the memory consumption of each node. On a 64-bit system,
    + * xa_node is currently 576 bytes, and we get 7 of them per 4kB page. If we
    + * doubled the number of slots per node, we'd get only 3 nodes per 4kB page.
    + */
    +#ifndef XA_CHUNK_SHIFT
    +#define XA_CHUNK_SHIFT (CONFIG_BASE_SMALL ? 4 : 6)
    +#endif
    +#define XA_CHUNK_SIZE (1UL << XA_CHUNK_SHIFT)
    +#define XA_CHUNK_MASK (XA_CHUNK_SIZE - 1)
    +
    +/*
    + * Internal entries have the bottom two bits set to the value 10b. Most
    + * internal entries are pointers to the next node in the tree. Since the
    + * kernel unmaps page 0 to trap NULL pointer dereferences, we can use values
    + * 0-1023 for special purposes. Values 0-62 are used for sibling
    + * entries. Value 256 is used for the retry entry.
    + */
    +
    +/* Private */
    +static inline void *xa_mk_internal(unsigned long v)
    +{
    + return (void *)((v << 2) | 2);
    +}
    +
    +/* Private */
    +static inline unsigned long xa_to_internal(void *entry)
    +{
    + return (unsigned long)entry >> 2;
    +}
    +
    +/**
    + * xa_is_internal() - Is the entry an internal entry?
    + * @entry: Entry retrieved from the XArray
    + *
    + * Return: %true if the entry is an internal entry.
    + */
    +static inline bool xa_is_internal(void *entry)
    +{
    + return ((unsigned long)entry & 3) == 2;
    +}
    +
    +/* Private */
    +static inline bool xa_is_node(void *entry)
    +{
    + return xa_is_internal(entry) && (unsigned long)entry > 4096;
    +}
    +
    +/* Private */
    +static inline void *xa_mk_sibling(unsigned int offset)
    +{
    + return xa_mk_internal(offset);
    +}
    +
    +/* Private */
    +static inline unsigned long xa_to_sibling(void *entry)
    +{
    + return xa_to_internal(entry);
    +}
    +
    +/**
    + * xa_is_sibling() - Is the entry a sibling entry?
    + * @entry: Entry retrieved from the XArray
    + *
    + * Return: %true if the entry is a sibling entry.
    + */
    +static inline bool xa_is_sibling(void *entry)
    +{
    + return IS_ENABLED(CONFIG_RADIX_TREE_MULTIORDER) &&
    + xa_is_internal(entry) &&
    + (entry < xa_mk_sibling(XA_CHUNK_SIZE - 1));
    +}
    +
    +#define XA_RETRY_ENTRY xa_mk_internal(256)
    +
    #endif /* _LINUX_XARRAY_H */
    diff --git a/lib/radix-tree.c b/lib/radix-tree.c
    index cda7a730e591..0a7a21dd9398 100644
    --- a/lib/radix-tree.c
    +++ b/lib/radix-tree.c
    @@ -37,6 +37,7 @@
    #include <linux/rcupdate.h>
    #include <linux/slab.h>
    #include <linux/string.h>
    +#include <linux/xarray.h>


    /* Number of nodes in fully populated tree of given height */
    @@ -97,24 +98,7 @@ static inline void *node_to_entry(void *ptr)
    return (void *)((unsigned long)ptr | RADIX_TREE_INTERNAL_NODE);
    }

    -#define RADIX_TREE_RETRY node_to_entry(NULL)
    -
    -#ifdef CONFIG_RADIX_TREE_MULTIORDER
    -/* Sibling slots point directly to another slot in the same node */
    -static inline
    -bool is_sibling_entry(const struct radix_tree_node *parent, void *node)
    -{
    - void __rcu **ptr = node;
    - return (parent->slots <= ptr) &&
    - (ptr < parent->slots + RADIX_TREE_MAP_SIZE);
    -}
    -#else
    -static inline
    -bool is_sibling_entry(const struct radix_tree_node *parent, void *node)
    -{
    - return false;
    -}
    -#endif
    +#define RADIX_TREE_RETRY XA_RETRY_ENTRY

    static inline unsigned long
    get_slot_offset(const struct radix_tree_node *parent, void __rcu **slot)
    @@ -128,16 +112,10 @@ static unsigned int radix_tree_descend(const struct radix_tree_node *parent,
    unsigned int offset = (index >> parent->shift) & RADIX_TREE_MAP_MASK;
    void __rcu **entry = rcu_dereference_raw(parent->slots[offset]);

    -#ifdef CONFIG_RADIX_TREE_MULTIORDER
    - if (radix_tree_is_internal_node(entry)) {
    - if (is_sibling_entry(parent, entry)) {
    - void __rcu **sibentry;
    - sibentry = (void __rcu **) entry_to_node(entry);
    - offset = get_slot_offset(parent, sibentry);
    - entry = rcu_dereference_raw(*sibentry);
    - }
    + if (xa_is_sibling(entry)) {
    + offset = xa_to_sibling(entry);
    + entry = rcu_dereference_raw(parent->slots[offset]);
    }
    -#endif

    *nodep = (void *)entry;
    return offset;
    @@ -299,10 +277,10 @@ static void dump_node(struct radix_tree_node *node, unsigned long index)
    } else if (!radix_tree_is_internal_node(entry)) {
    pr_debug("radix entry %p offset %ld indices %lu-%lu parent %p\n",
    entry, i, first, last, node);
    - } else if (is_sibling_entry(node, entry)) {
    + } else if (xa_is_sibling(entry)) {
    pr_debug("radix sblng %p offset %ld indices %lu-%lu parent %p val %p\n",
    entry, i, first, last, node,
    - *(void **)entry_to_node(entry));
    + node->slots[xa_to_sibling(entry)]);
    } else {
    dump_node(entry_to_node(entry), first);
    }
    @@ -872,8 +850,7 @@ static void radix_tree_free_nodes(struct radix_tree_node *node)

    for (;;) {
    void *entry = rcu_dereference_raw(child->slots[offset]);
    - if (radix_tree_is_internal_node(entry) &&
    - !is_sibling_entry(child, entry)) {
    + if (xa_is_node(entry)) {
    child = entry_to_node(entry);
    offset = 0;
    continue;
    @@ -895,7 +872,7 @@ static void radix_tree_free_nodes(struct radix_tree_node *node)
    static inline int insert_entries(struct radix_tree_node *node,
    void __rcu **slot, void *item, unsigned order, bool replace)
    {
    - struct radix_tree_node *child;
    + void *sibling;
    unsigned i, n, tag, offset, tags = 0;

    if (node) {
    @@ -913,7 +890,7 @@ static inline int insert_entries(struct radix_tree_node *node,
    offset = offset & ~(n - 1);
    slot = &node->slots[offset];
    }
    - child = node_to_entry(slot);
    + sibling = xa_mk_sibling(offset);

    for (i = 0; i < n; i++) {
    if (slot[i]) {
    @@ -930,7 +907,7 @@ static inline int insert_entries(struct radix_tree_node *node,
    for (i = 0; i < n; i++) {
    struct radix_tree_node *old = rcu_dereference_raw(slot[i]);
    if (i) {
    - rcu_assign_pointer(slot[i], child);
    + rcu_assign_pointer(slot[i], sibling);
    for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++)
    if (tags & (1 << tag))
    tag_clear(node, tag, offset + i);
    @@ -940,9 +917,7 @@ static inline int insert_entries(struct radix_tree_node *node,
    if (tags & (1 << tag))
    tag_set(node, tag, offset);
    }
    - if (radix_tree_is_internal_node(old) &&
    - !is_sibling_entry(node, old) &&
    - (old != RADIX_TREE_RETRY))
    + if (xa_is_node(old))
    radix_tree_free_nodes(old);
    if (xa_is_value(old))
    node->exceptional--;
    @@ -1101,10 +1076,10 @@ static inline void replace_sibling_entries(struct radix_tree_node *node,
    void __rcu **slot, int count, int exceptional)
    {
    #ifdef CONFIG_RADIX_TREE_MULTIORDER
    - void *ptr = node_to_entry(slot);
    - unsigned offset = get_slot_offset(node, slot) + 1;
    + unsigned offset = get_slot_offset(node, slot);
    + void *ptr = xa_mk_sibling(offset);

    - while (offset < RADIX_TREE_MAP_SIZE) {
    + while (++offset < RADIX_TREE_MAP_SIZE) {
    if (rcu_dereference_raw(node->slots[offset]) != ptr)
    break;
    if (count < 0) {
    @@ -1112,7 +1087,6 @@ static inline void replace_sibling_entries(struct radix_tree_node *node,
    node->count--;
    }
    node->exceptional += exceptional;
    - offset++;
    }
    #endif
    }
    @@ -1311,8 +1285,7 @@ int radix_tree_split(struct radix_tree_root *root, unsigned long index,
    tags |= 1 << tag;

    for (end = offset + 1; end < RADIX_TREE_MAP_SIZE; end++) {
    - if (!is_sibling_entry(parent,
    - rcu_dereference_raw(parent->slots[end])))
    + if (!xa_is_sibling(rcu_dereference_raw(parent->slots[end])))
    break;
    for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++)
    if (tags & (1 << tag))
    @@ -1608,11 +1581,9 @@ static void set_iter_tags(struct radix_tree_iter *iter,
    static void __rcu **skip_siblings(struct radix_tree_node **nodep,
    void __rcu **slot, struct radix_tree_iter *iter)
    {
    - void *sib = node_to_entry(slot - 1);
    -
    while (iter->index < iter->next_index) {
    *nodep = rcu_dereference_raw(*slot);
    - if (*nodep && *nodep != sib)
    + if (*nodep && !xa_is_sibling(*nodep))
    return slot;
    slot++;
    iter->index = __radix_tree_iter_add(iter, 1);
    @@ -1763,7 +1734,7 @@ void __rcu **radix_tree_next_chunk(const struct radix_tree_root *root,
    while (++offset < RADIX_TREE_MAP_SIZE) {
    void *slot = rcu_dereference_raw(
    node->slots[offset]);
    - if (is_sibling_entry(node, slot))
    + if (xa_is_sibling(slot))
    continue;
    if (slot)
    break;
    --
    2.15.0
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