idr and radix-tree

idr用到了radix-tree。如果只有一个节点数据结构如下：

crash> idr  ffff880b4d726dd0struct idr {  idr_rt = {    gfp_mask = 0x6000000,    rnode = 0xffff880bd74d7909  },  idr_next = 0x0}crash> radix_tree_node  ffff880bd74d7908struct radix_tree_node {  shift = 0x0,  offset = 0x0,  count = 0x1,  exceptional = 0x0,  parent = 0x0,  root = 0xffff880b4d726dd0,  {    private_list = {      next = 0xffff880bd74d7920,      prev = 0xffff880bd74d7920    },    callback_head = {      next = 0xffff880bd74d7920,      func = 0xffff880bd74d7920    }  },  slots = {0x0, 0xffff880b4d726000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},  tags = {{0xfffffffffffffffd}, {0x0}, {0x0}}}

这里有几点需要注意。第一，如果node末位是1，说明不是叶子节点。dump的时候一定要把1去掉。

#define RADIX_TREE_ENTRY_MASK           3UL#define RADIX_TREE_INTERNAL_NODE        1ULstatic inline bool radix_tree_is_internal_node(void *ptr){        return ((unsigned long)ptr & RADIX_TREE_ENTRY_MASK) ==                                RADIX_TREE_INTERNAL_NODE;}

第二，count就是孩子的个数，包括中间节点和叶子节点。
第三，tags就是bitmap，是叶子节点置为0。否则为１。


再举个例子63个节点的例子：

crash> radix_tree_node  ffff880bc1dc16d0struct radix_tree_node {  shift = 0x0,  offset = 0x0,  count = 0x3f,  exceptional = 0x0,  parent = 0x0,  root = 0xffff880b5354add0,  {    private_list = {      next = 0xffff880bc1dc16e8,      prev = 0xffff880bc1dc16e8    },    callback_head = {      next = 0xffff880bc1dc16e8,      func = 0xffff880bc1dc16e8    }  },  slots = {0x0, 0xffff880b5354aa00, 0xffff880b5354b800, 0xffff880b5354ba00, 0xffff880b5354b600, 0xffff880b53548600, 0xffff880b5354b200, 0xffff880b53549a00, 0xffff880b5354a800, 0xffff880b53548400, 0xffff880b5354bc00, 0xffff880b5354b400, 0xffff880b5354be00, 0xffff880b5354ae00, 0xffff880b53549400, 0xffff880b53549e00, 0xffff880b53549200, 0xffff880b5354b000, 0xffff880b53549c00, 0xffff880b5354a400, 0xffff880b53548800, 0xffff880b53549800, 0xffff880b5354a000, 0xffff880b53549000, 0xffff880b53548c00, 0xffff880b53548200, 0xffff880b5354a200, 0xffff880b53548a00, 0xffff880b9a415600, 0xffff880b9a417600, 0xffff880b9a415800, 0xffff880b9a417200, 0xffff880b9a417800, 0xffff880b9a414800, 0xffff880b9a414200, 0xffff880b9a414000, 0xffff880b9a414e00, 0xffff880b9a415000, 0xffff880b9a417400, 0xffff880b9a414600, 0xffff880b9a416400, 0xffff880b9a414c00, 0xffff880b9a414a00, 0xffff880b9a415200, 0xffff880b5e3f7c00, 0xffff880b5e3f5400, 0xffff880b5e3f4400, 0xffff880b5e3f6e00, 0xffff880b5e3f5e00, 0xffff880b5e3f5c00, 0xffff880b5e3f7a00, 0xffff880b5e3f4c00, 0xffff880b5e3f6200, 0xffff880b5e3f5800, 0xffff880b5e3f6000, 0xffff880b5e3f7200, 0xffff880b5e3f6800, 0xffff880b5e3f5a00, 0xffff880b5e3f6400, 0xffff880b5e3f5000, 0xffff880b5e3f6c00, 0xffff880b5e3f4800, 0xffff880b5c46e000, 0xffff880b5c46ea00},  tags = {{0x1}, {0x0}, {0x0}}}

那么offset是什么的？首先创建有200个节点的树。

crash> radix_tree_node  ffff880bd74d76c0struct radix_tree_node {  shift = 0x6,  offset = 0x0,  count = 0x4,  exceptional = 0x0,  parent = 0x0,  root = 0xffff880b4d7247d0,  {    private_list = {      next = 0xffff880bd74d76d8,      prev = 0xffff880bd74d76d8    },    callback_head = {      next = 0xffff880bd74d76d8,      func = 0xffff880bd74d76d8    }  },  slots = {0xffff880bd74d7b51, 0xffff880bd74d7479, 0xffff880bd74d6da1, 0xffff880bd74d6fe9, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},  tags = {{0xfffffffffffffff9}, {0x0}, {0x0}}}crash> radix_tree_node.offset  0xffff880bd74d7b50  offset = 0x0crash> radix_tree_node.offset  0xffff880bd74d7478  offset = 0x1crash> radix_tree_node.offset  0xffff880bd74d6da0  offset = 0x2crash> radix_tree_node.offset  0xffff880bd74d6fe8  offset = 0x3

应该可以理解为孩子的编号，从0开始。