Linux dm中minor number的管理——idr

jack_zheng

                1. idr机制
Linux的dm中对minor number采用idr机制进行管理。idr即"ID Radix",内核中通过radix树对ID进行组织和管理，是一种将整数ID和指针关联在一起的一种机制。radix树基于以二进制表示的键值的查找树，尤其适合于处理非常长的、可变长度的键值。查找时每个节点都存储有进行下一次的bit测试之前需要跳过的bit数目，查找效率比较高。
以下代码中的注释说明了idr的基本作用（lib/idr.c）
/*
* 2002-10-18  written by Jim Houston jim.houston@ccur.com
*  Copyright (C) 2002 by Concurrent Computer Corporation
*  Distributed under the GNU GPL license version 2.
*
* Modified by George Anzinger to reuse immediately and to use
* find bit instructions.  Also removed _irq on spinlocks.
*
* Small id to pointer translation service.  
*
* It uses a radix tree like structure as a sparse array indexed
* by the id to obtain the pointer.  The bitmap makes allocating
* a new id quick.  
*
* You call it to allocate an id (an int) an associate with that id a
* pointer or what ever, we treat it as a (void *).  You can pass this
* id to a user for him to pass back at a later time.  You then pass
* that id to this code and it returns your pointer.
   
* You can release ids at any time. When all ids are released, most of
* the memory is returned (we keep IDR_FREE_MAX) in a local pool so we
* don't need to go to the memory "store" during an id allocate, just
* so you don't need to be too concerned about locking and conflicts
* with the slab allocator.
*/
API函数主要是以下几个：
void *idr_find(struct idr *idp, int id);
int idr_pre_get(struct idr *idp, unsigned gfp_mask);
int idr_get_new(struct idr *idp, void *ptr, int *id);
int idr_get_new_above(struct idr *idp, void *ptr, int starting_id, int *id);
void idr_remove(struct idr *idp, int id);
void idr_init(struct idr *idp);
/**
* idr_get_new - allocate new idr entry
* @idp: idr handle
* @ptr: pointer you want associated with the ide
* @id: pointer to the allocated handle
*
* This is the allocate id function.  It should be called with any
* required locks.
*  
* If memory is required, it will return -EAGAIN, you should unlock
* and go back to the idr_pre_get() call.  If the idr is full, it will
* return -ENOSPC.
*
* @id returns a value in the range 0 ... 0x7fffffff
*/
int idr_get_new(struct idr *idp, void *ptr, int *id)
{   
    int rv;
    rv = idr_get_new_above_int(idp, ptr, 0);
    /*
     * This is a cheap hack until the IDR code can be fixed to
     * return proper error values.
     */
    if (rv disk = alloc_disk(1);
    md->disk->major = _major;
    md->disk->first_minor = minor;
    md->disk->queue = md->queue;
    sprintf(md->disk->disk_name, "dm-%d", minor);
    add_disk(md->disk);
    init_waitqueue_head(&md->wait);
    init_waitqueue_head(&md->eventq);
}
1）对于指定minor
/*
* See if the device with a specific minor # is free.
*/
static int specific_minor(unsigned int minor)
{
    int r, m;
    if (minor >= (1 = (1 name);
    if (r)   
        return r;
   
    if (param->flags & DM_PERSISTENT_DEV_FLAG)
        r = dm_create_with_minor(MINOR(huge_decode_dev(param->dev)), &md);
    else
        r = dm_create(&md);
}
               
               
               
               
               

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