Linux块设备可以分为三类。分别针对顺序访问物理设备、随机访问物理设备和逻辑设备（即“栈式设备”）
| 类型 | make_request_fn | request_fn | 备注 |
| SCSI 设备等 | 从bio构造request（经过合并和排序），返回0 | 逐个处理request | 调用blk_init_queue，使用默认的__make_request，提供策略例程 |
| SSD等 | 直接处理bio，返回0 | 无 | 调用blk_alloc_queue，提供make_request_fn |
| RAID或Device Mapper设备 | 重定向bio，返回非零值 | 无 | 调用blk_alloc_queue，提供make_request_fn |
blk_init_queue原型：
struct request_queue *blk_init_queue(request_fn_proc *rfn, spinlock_t *lock)
{
return blk_init_queue_node(rfn, lock, NUMA_NO_NODE);
}
EXPORT_SYMBOL(blk_init_queue); struct request_queue *
blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
{
struct request_queue *uninit_q, *q;
uninit_q = blk_alloc_queue_node(GFP_KERNEL, node_id);
if (!uninit_q)
return NULL;
q = blk_init_allocated_queue(uninit_q, rfn, lock);
if (!q)
blk_cleanup_queue(uninit_q);
return q;
}
EXPORT_SYMBOL(blk_init_queue_node); struct request_queue *
blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
spinlock_t *lock)
{
if (!q)
return NULL;
if (blk_init_rl(&q->root_rl, q, GFP_KERNEL))
return NULL;
q->request_fn = rfn;
q->prep_rq_fn = NULL;
q->unprep_rq_fn = NULL;
q->queue_flags |= QUEUE_FLAG_DEFAULT;
/* Override internal queue lock with supplied lock pointer */
if (lock)
q->queue_lock = lock;
/*
* This also sets hw/phys segments, boundary and size
*/
blk_queue_make_request(q, blk_queue_bio); //使用blk_init_queue会默认绑定blk_queue_bio来处理IO
q->sg_reserved_size = INT_MAX;
/* init elevator */
if (elevator_init(q, NULL)) // 初始化IO调度
return NULL;
return q;
}
EXPORT_SYMBOL(blk_init_allocated_queue);
下面来跟踪blk_queue_bio函数：
void blk_queue_bio(struct request_queue *q, struct bio *bio)
{
const bool sync = !!(bio->bi_rw & REQ_SYNC);
struct blk_plug *plug;
int el_ret, rw_flags, where = ELEVATOR_INSERT_SORT;
struct request *req;
unsigned int request_count = ;
/*
* low level driver can indicate that it wants pages above a
* certain limit bounced to low memory (ie for highmem, or even
* ISA dma in theory)
*/
blk_queue_bounce(q, &bio); // 如果需要，创建反弹缓冲区
if (bio_integrity_enabled(bio) && bio_integrity_prep(bio)) {
bio_endio(bio, -EIO);
return;
}
if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
spin_lock_irq(q->queue_lock);
where = ELEVATOR_INSERT_FLUSH;
goto get_rq;
}
/*
* Check if we can merge with the plugged list before grabbing any locks
* 首先尝试请求合并
*/
if (attempt_plug_merge(q, bio, &request_count))
return;
spin_lock_irq(q->queue_lock);
el_ret = elv_merge(q, &req, bio); // 判断是否bio是否可以合并
// 如果可以合并的话，分为向前和向后合并
if (el_ret == ELEVATOR_BACK_MERGE) {
if (bio_attempt_back_merge(q, req, bio)) {
elv_bio_merged(q, req, bio); // 请求如果在硬件上允许，则进行合并
if (!attempt_back_merge(q, req)) // 合并之后可能两个request可以合并
elv_merged_request(q, req, el_ret);
goto out_unlock;
}
} else if (el_ret == ELEVATOR_FRONT_MERGE) {
if (bio_attempt_front_merge(q, req, bio)) {
elv_bio_merged(q, req, bio);
if (!attempt_front_merge(q, req))
elv_merged_request(q, req, el_ret);
goto out_unlock;
}
}
// 不能合并就根据bio构造request
get_rq:
/*
* This sync check and mask will be re-done in init_request_from_bio(),
* but we need to set it earlier to expose the sync flag to the
* rq allocator and io schedulers.
*/
rw_flags = bio_data_dir(bio);
if (sync)
rw_flags |= REQ_SYNC;
/*
* Grab a free request. This is might sleep but can not fail.
* Returns with the queue unlocked.
*/
req = get_request(q, rw_flags, bio, GFP_NOIO); // 获取一个request
if (unlikely(!req)) {
bio_endio(bio, -ENODEV); /* @q is dead */
goto out_unlock;
}
/*
* After dropping the lock and possibly sleeping here, our request
* may now be mergeable after it had proven unmergeable (above).
* We don't worry about that case for efficiency. It won't happen
* often, and the elevators are able to handle it.
*/
init_request_from_bio(req, bio); // 根据bio构造一个request，并添加到IO调度器队列
if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags))
req->cpu = raw_smp_processor_id();
plug = current->plug;
// 接下来是蓄流/泄流策略
if (plug) {
/*
* If this is the first request added after a plug, fire
* of a plug trace. If others have been added before, check
* if we have multiple devices in this plug. If so, make a
* note to sort the list before dispatch.
*/
if (list_empty(&plug->list))
trace_block_plug(q);
else {
if (request_count >= BLK_MAX_REQUEST_COUNT) {
blk_flush_plug_list(plug, false);
trace_block_plug(q);
}
}
list_add_tail(&req->queuelist, &plug->list);
drive_stat_acct(req, );
} else {
spin_lock_irq(q->queue_lock);
add_acct_request(q, req, where); // 将请求添加到IO调度队列或请求队列，主要被用来处理屏障请求
__blk_run_queue(q);
out_unlock:
spin_unlock_irq(q->queue_lock);
}
}
EXPORT_SYMBOL_GPL(blk_queue_bio); /* for device mapper only */
第13行，blk_queue_bounce创建一个反弹缓冲区。通常是在驱动尝试在外围设备不可达到的地址。例如高端内存上执行DMA等。创建反弹缓冲区后，数据要在原缓冲区和反弹缓冲区之间进行与读写方向对应的复制。毫无疑问，使用反弹缓冲区会降低性能，但也没有其他办法。
所谓反弹，实际上是分配一个新的bio描述符，它和原始bio的segment一一对应。如果原始bio的segment使用的页面在DMA内存范围外，则分配一个在DMA范围内的页面，赋给新的bio对应的segment。对于写操作，需要将旧bio页面的内容复制到新的bio中。如果原始的bio的segment使用的页面在DMA范围内，则将新的bio指向同一地方。
最后将原始bio保存在新的bio的bi_private域中，并设置新bio的完成回调函数。
接下来交给IO调度器，由它负责合并和排序请求。合并是指将对磁盘上连续位置的请求合并为一个，通过一次SCSI命令完成。排序是将多个请求对磁盘上的访问位置顺序重新排列，使得磁头尽可能向一个方向移动。请求的合并和排序是在SCSI设备的请求队列描述符上进行的。
int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
{
struct elevator_queue *e = q->elevator;
struct request *__rq;
int ret; /*
* Levels of merges:
* nomerges: No merges at all attempted
* noxmerges: Only simple one-hit cache try
* merges: All merge tries attempted
*/
if (blk_queue_nomerges(q)) // 如果设置了QUEUE_FLAG_NOMERGES的标志位，就直接返回不合并
return ELEVATOR_NO_MERGE; /*
* First try one-hit cache.
*/
// 如果请求队列的last_merge有缓存下来的request，调用blk_try_merge来进行尝试和它进行合并，如果可以合并，通过参数输出这个req
if (q->last_merge && elv_rq_merge_ok(q->last_merge, bio)) {
ret = blk_try_merge(q->last_merge, bio);
if (ret != ELEVATOR_NO_MERGE) {
*req = q->last_merge;
return ret;
}
} // 如果设置了QUEUE_FLAG_NOXMERGES的标志位，表明不要进行“扩展”的合并尝试
if (blk_queue_noxmerges(q))
return ELEVATOR_NO_MERGE; /*
* See if our hash lookup can find a potential backmerge.
* 后面的代码就是所谓的“扩展”合并尝试，它包含两方面的内容：
* 第一部分是各种IO调度算法全都适用的，而第二部分则是各种IO调度算法特定的
*/
__rq = elv_rqhash_find(q, bio->bi_sector);
if (__rq && elv_rq_merge_ok(__rq, bio)) {
*req = __rq;
return ELEVATOR_BACK_MERGE;
} /*
* IO调度特定的合并算法是通过电梯队列操作表的elevator_merge_fn回调实现的
*/
if (e->type->ops.elevator_merge_fn)
return e->type->ops.elevator_merge_fn(q, req, bio); return ELEVATOR_NO_MERGE;
}
如果我们的请求不能合并到现有的request中，那么就要新申请request描述符了，根据bio对它初始化，并添加到IO调度器队列
最后Linux块设备层采用蓄流/泄流技术来改进吞吐量，蓄流是为了将请求合并和排序，然后一起泄流，泄流函数为__blk_run_queue(q)
/**
* __blk_run_queue - run a single device queue
* @q: The queue to run
*
* Description:
* See @blk_run_queue. This variant must be called with the queue lock
* held and interrupts disabled.
*/
void __blk_run_queue(struct request_queue *q)
{
if (unlikely(blk_queue_stopped(q)))
return;
__blk_run_queue_uncond(q);
} /**
* __blk_run_queue_uncond - run a queue whether or not it has been stopped
* @q: The queue to run
*
* Description:
* Invoke request handling on a queue if there are any pending requests.
* May be used to restart request handling after a request has completed.
* This variant runs the queue whether or not the queue has been
* stopped. Must be called with the queue lock held and interrupts
* disabled. See also @blk_run_queue.
*/
inline void __blk_run_queue_uncond(struct request_queue *q)
{
if (unlikely(blk_queue_dead(q)))
return;
/*
* Some request_fn implementations, e.g. scsi_request_fn(), unlock
* the queue lock internally. As a result multiple threads may be
* running such a request function concurrently. Keep track of the
* number of active request_fn invocations such that blk_drain_queue()
* can wait until all these request_fn calls have finished.
*/
q->request_fn_active++;
q->request_fn(q); // 回调函数实例化为scsi_request_fn，也就是通常所说的SCSI策略例程
q->request_fn_active--;
}
__blk_run_queue
对于SCSI设备，在为它分配请求队列时，将请求队列的request_fn回调函数实例化为scsi_request_fn，也就是通常所说的SCSI策略例程。