全部博文（86）
分类： LINUX
2015-08-07 16:46:00
转自: http://blog.csdn.net/vanbreaker/article/details/8275264
在通用块层中，bio用来描述单一的I/O请求，它记录了一次I/O操作所必需的相关信息，如用于I/O操作的数据缓存位置，I/O操作的块设备起始扇区，是读操作还是写操作等等。struct bio的定义如下
struct bio {
sector_t bi_sector; /* device address in 512 byte
sectors */
struct bio *bi_next; /* request queue link */
struct block_device *bi_bdev;
unsigned long bi_flags; /* status, command, etc */
unsigned long bi_rw; /* bottom bits READ/WRITE,
* top bits priority
*/
unsigned short bi_vcnt; /* how many bio_vec's */
unsigned short bi_idx; /* current index into bvl_vec */
/* Number of segments in this BIO after
* physical address coalescing is performed.
*/
unsigned int bi_phys_segments;
unsigned int bi_size; /* residual I/O count */
/*
* To keep track of the max segment size, we account for the
* sizes of the first and last mergeable segments in this bio.
*/
unsigned int bi_seg_front_size;
unsigned int bi_seg_back_size;
unsigned int bi_max_vecs; /* max bvl_vecs we can hold */
unsigned int bi_comp_cpu; /* completion CPU */
atomic_t bi_cnt; /* pin count */
struct bio_vec *bi_io_vec; /* the actual vec list */
bio_end_io_t *bi_end_io;
void *bi_private;
#if defined(CONFIG_BLK_DEV_INTEGRITY)
struct bio_integrity_payload;*bi_integrity; /* data integrity */
#endif
bio_destructor_t *bi_destructor; /* destructor */
/*
* We can inline a number of vecs at the end of the bio, to avoid
* double allocations for a small number of bio_vecs. This member
* MUST obviously be kept at the very end of the bio.
*/
struct bio_vec bi_inline_vecs[0];
};
bi_sector:该I/O操作的起始扇区号
bi_rw:指明了读写方向
bi_vcnt:该I/O操作中涉及到了多少个缓存向量，每个缓存向量由[page,offset,len]来描述
bi_idx:指示当前的缓存向量
bi_io_vec：缓存向量数组
缓存向量的定义：
struct bio_vec {
struct page *bv_page;
unsigned int bv_len;
unsigned int bv_offset;
};
struct request用于描述提交给块设备的I/O请求，bio会动态地添加进request，因此一个request往往会包含若干相邻的bio。
struct request {
struct list_head queuelist;
struct call_single_data csd;
int cpu;
struct request_queue *q;
unsigned int cmd_flags;
enum rq_cmd_type_bits cmd_type;
unsigned long atomic_flags;
/* the following two fields are internal, NEVER access directly */
sector_t __sector; /* sector cursor */
unsigned int __data_len; /* total data len */
struct bio *bio;
struct bio *biotail;
struct hlist_node hash; /* merge hash */
/*
* The rb_node is only used inside the io scheduler, requests
* are pruned when moved to the dispatch queue. So let the
* completion_data share space with the rb_node.
*/
union {
struct rb_node rb_node; /* sort/lookup */
void *completion_data;
};
/*
* two pointers are available for the IO schedulers, if they need
* more they have to dynamically allocate it.
*/
void *elevator_private;
void *elevator_private2;
struct gendisk *rq_disk;
unsigned long start_time;
/* Number of scatter-gather DMA addr+len pairs after
* physical address coalescing is performed.
*/
unsigned short nr_phys_segments;
unsigned short ioprio;
void *special; /* opaque pointer available for LLD use */
char *buffer; /* kaddr of the current segment if available */
int tag;
int errors;
int ref_count;
/*
* when request is used as a packet command carrier
*/
unsigned short cmd_len;
unsigned char __cmd[BLK_MAX_CDB];
unsigned char *cmd;
unsigned int extra_len; /* length of alignment and padding */
unsigned int sense_len;
unsigned int resid_len; /* residual count */
void *sense;
unsigned long deadline;
struct list_head timeout_list;
unsigned int timeout;
int retries;
/*
* completion callback.
*/
rq_end_io_fn *end_io;
void *end_io_data;
/* for bidi */
struct request *next_rq;
};
queuelist:用于将request链入请求队列的链表元素
q:指向所属的请求队列
__sector:下一个要传输的bio的起始扇区号
__data_len:request要传输的数据字节数
bio,biotail:用于维护request中的bio链表
在之前介绍的gendisk结构中，可以看到每个块设备(或分区)都对应了一个request_queue的结构，该结构用来容纳request,并且包含了相应的递交request以及I/O调度的方法
递交一个bio的主要工作是从generic_make_request()函数开始的，我们以此为入口来分析一个bio的递交过程。在每个进程的task_struct中，都包含有两个变量----struct bio *bio_list, **bio_tail，generic_make_request()的主要工作就是用这两个变量来维护当前待添加的bio链表，实际的提交操作会由generic_make_request()调用__generic_make_request()函数完成。而在__generic_make_request()中，会调用到queue_list中定义的make_request_fn函数，也就是特定于设备的提交请求函数来完成后续的工作。在这里便会有一些问题，大部分设备的make_request_fn都可以直接定义为内核实现的__make_request函数，而一些设备需要使用自己的make_request_fn，而自行实现的make_request_fn有可能会递归调用gerneric_make_request(),由于内核的堆栈十分有限，因此在generic_make_request()的实现中，玩了一些小把戏，使得递归的深度不会超过一层。我们注意到bio_tail是一个二级指针，这个值最初是NULL，当有bio添加进来，bio_tail将会指向bio->bi_next(如果bio全都递交上去了，则bio_tail将会指向bio_list)，也就是说除了第一次调用外，其他每次递归调用generic_make_request()函数都会出现bio_tail不为NULL的情形，因此当bio_tail不为NULL时，则只将bio添加到由bio_list和bio_tail维护的链表中，然后直接返回，而不调用__generic_make_request()，这样便防止了多重递归的产生
void generic_make_request(struct bio *bio)
{
if (current->bio_tail) {//current->bio_tail不为空则表明有bio正在提交,也就是说是处于递归调用
/* make_request is active */
bio->bi_next = NULL;
/*这里current->tail有两种情况，当current的bio链表为空时，bio_tail指向的是bio_list
当current的bio链表不为空时，bio_tail指向的是最后一个bio的bi_next指针，因此
这句的实际作用就是将bio添加到了current的bio链表的尾部*/
*(current->bio_tail) = bio;
current->bio_tail = &bio->bi_next;
/*这里直接返回，遍历并且提交bio的工作永远都是交给最先调用的generic_make_request来处理的，避免了多重递归*/
return;
}
/* following loop may be a bit non-obvious, and so deserves some
* explanation.
* Before entering the loop, bio->bi_next is NULL (as all callers
* ensure that) so we have a list with a single bio.
* We pretend that we have just taken it off a longer list, so
* we assign bio_list to the next (which is NULL) and bio_tail
* to &bio_list, thus initialising the bio_list of new bios to be
* added. __generic_make_request may indeed add some more bios
* through a recursive call to generic_make_request. If it
* did, we find a non-NULL value in bio_list and re-enter the loop
* from the top. In this case we really did just take the bio
* of the top of the list (no pretending) and so fixup bio_list and
* bio_tail or bi_next, and call into __generic_make_request again.
*
* The loop was structured like this to make only one call to
* __generic_make_request (which is important as it is large and
* inlined) and to keep the structure simple.
*/
BUG_ON(bio->bi_next);
do {
current->bio_list = bio->bi_next;//这里取current的待提交bio链表的下一个bio
if (bio->bi_next == NULL)//bi_next为空，也就是说待提交链表已经空了，只剩下最后一个bio了
current->bio_tail = ¤t->bio_list;//bio_tail指向bio_list
else
bio->bi_next = NULL;//否则将bio提取出来
__generic_make_request(bio);//提交bio
bio = current->bio_list;//取新的待提交bio
} while (bio);
current->bio_tail = NULL; /* deactivate */
}
__generic_make_request()首先由bio对应的block_device获取等待队列q，然后要检查对应的设备是不是分区，如果是分区的话要将扇区地址进行重新计算，最后调用make_request_fn完成bio的递交
static inline void __generic_make_request(struct bio *bio)
{
struct request_queue *q;
sector_t old_sector;
int ret, nr_sectors = bio_sectors(bio);//提取bio的大小，以扇区为单位
dev_t old_dev;
int err = -EIO;
might_sleep();
//这里检查bio的传输起始扇区是否超过设备的最大扇区，并且两者之间的差不能小于nr_sector
if (bio_check_eod(bio, nr_sectors))
goto end_io;
/*
* Resolve the mapping until finished. (drivers are
* still free to implement/resolve their own stacking
* by explicitly returning 0)
*
* NOTE: we don't repeat the blk_size check for each new device.
* Stacking drivers are expected to know what they are doing.
*/
old_sector = -1;
old_dev = 0;
do {
char b[BDEVNAME_SIZE];
q = bdev_get_queue(bio->bi_bdev);//获取对应设备的请求队列
if (unlikely(!q)) {
printk(KERN_ERR
"generic_make_request: Trying to access "
"nonexistent block-device %s (%Lu)\n",
bdevname(bio->bi_bdev, b),
(long long) bio->bi_sector);
goto end_io;
}
/*下面做一些必要的检查*/
if (unlikely(!bio_rw_flagged(bio, BIO_RW_DISCARD) &&
nr_sectors > queue_max_hw_sectors(q))) {
printk(KERN_ERR "bio too big device %s (%u > %u)\n",
bdevname(bio->bi_bdev, b),
bio_sectors(bio),
queue_max_hw_sectors(q));
goto end_io;
}
if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
goto end_io;
if (should_fail_request(bio))
goto end_io;
/*
* If this device has partitions, remap block n
* of partition p to block n+start(p) of the disk.
*/
//如果bio指定的是一个分区，则传输点要重新进行计算
blk_partition_remap(bio);
if (bio_integrity_enabled(bio) && bio_integrity_prep(bio))
goto end_io;
if (old_sector != -1)
trace_block_remap(q, bio, old_dev, old_sector);
old_sector = bio->bi_sector;
?ld_dev = bio->bi_bdev->bd_dev;
if (bio_check_eod(bio, nr_sectors))
goto end_io;
if (bio_rw_flagged(bio, BIO_RW_DISCARD) &&
!blk_queue_discard(q)) {
err = -EOPNOTSUPP;
goto end_io;
}
trace_block_bio_queue(q, bio);
ret = q->make_request_fn(q, bio);//这里是关键，调用请求队列中的make_request_fn函数处理请求
} while (ret);
return;
end_io:
bio_endio(bio, err);
}
辅助函数blk_partition_remap():
static inline void blk_partition_remap(struct bio *bio)
{
struct block_device *bdev = bio->bi_bdev;
/*首先要保证传输的大小不能小于1个扇区并且bdev确实是分区*/
if (bio_sectors(bio) && bdev != bdev->bd_contains) {
struct hd_struct *p = bdev->bd_part;//获取分区信息
bio->bi_sector += p->start_sect;//在传输起点的原基础上加上分区的起始扇区号
bio->bi_bdev = bdev->bd_contains;//将bio的bdev置为主设备
trace_block_remap(bdev_get_queue(bio->bi_bdev), bio,
bdev->bd_dev,
bio->bi_sector - p->start_sect);
}
}
可以看到这里将bio的参考对象设置为了主设备，而不是分区，因此对应的扇区起始号也要计算为扇区的绝对值。
大多数的make_request_fn函数都可以直接定义为__make_request(),我们通过这个函数来分析递交bio的关键操作
static int __make_request(struct request_queue *q, struct bio *bio)
{
struct request *req;
int el_ret;
unsigned int bytes = bio->bi_size;
const unsigned short prio = bio_prio(bio);
const bool sync = bio_rw_flagged(bio, BIO_RW_SYNCIO);
const bool unplug = bio_rw_flagged(bio, BIO_RW_UNPLUG);
const unsigned int ff = bio->bi_rw & REQ_FAILFAST_MASK;
int rw_flags;
/*如果BIO_RW_BARRIER被置位(表示必须得让请求队列中的所有bio传递完毕才处理自己)，
但是不支持hardbarrier，不能进行bio的提交*/
if (bio_rw_flagged(bio, BIO_RW_BARRIER) &&
(q->next_ordered == QUEUE_ORDERED_NONE)) {
bio_endio(bio, -EOPNOTSUPP);
return 0;
}
/*
* 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);
spin_lock_irq(q->queue_lock);
//如果BIO_RW_BARRIER被置位或者请求队列为空，则情况比较简单，不用进行bio的合并，跳转到get_rq处处理
if (unlikel}(bio_rw_flagged(bio, BIO_RW_BARRIER)) || elv_queue_empty(q))
goto get_rq;
/**请求队列不为空**/
/*elv_merge()试图寻找一个已存在的request,将bio并入其中*/
el_ret = elv_merge(q, &req, bio);
switch (el_ret) {
case ELEVATOR_BACK_MERGE:
BUG_ON(!rq_mergeable(req));
/*相关检查*/
if (!ll_back_merge_fn(q, req, bio))
break;
trace_block_bio_backmerge(q, bio);
if ((req->cmd_flags & REQ_FAILFAST_MASK) != ff)
blk_rq_set_mixed_merge(req);
/*这里将bio插入到request尾部*/
req->biotail->bi_next = bio;
req->biotail = bio;
req->__data_len += bytes;
req->ioprio = ioprio_best(req->ioprio, prio);
if (!blk_rq_cpu_valid(req))
req->cpu = bio->bi_comp_cpu;
drive_stat_acct(req, 0);
if (!attempt_back_merge(q, req))
elv_merged_request(q, req, el_ret);
goto out;
case ELEVATOR_FRONT_MERGE:
BUG_ON(!rq_mergeable(req));
if (!ll_front_merge_fn(q, req, bio))
break;
trace_block_bio_frontmerge(q, bio);
if ((req->cmd_flags & REQ_FAILFAST_MASK) != ff) {
blk_rq_set_mixed_merge(req);
req->cmd_flags &= ~REQ_FAILFAST_MASK;
req->cmd_flags |= ff;
}
/*这里将bio插入到request的头部*/
bio->bi_next = req->bio;
req->bio = bio;
/*
* may not be valid. if the low level driver said
* it didn't need a bounce buffer then it better
* not touch req->buffer either...
*/
req->buffer = bio_data(bio);
req->__sector = bio->bi_sector;
req->__data_len += bytes;
req->ioprio = ioprio_best(req->ioprio, prio);
if (!blk_rq_cpu_valid(req))
req->cpu = bio->bi_comp_cpu;
drive_stat_acct(req, 0);
if (!attempt_front_merge(q, req))
elv_merged_request(q, req, el_ret);
goto out;
/* ELV_NO_MERGE: elevator says don't/can't merge. */
default:
;
}
get_rq:/**下面的代码对应请求队列为空的情况，5E9??要先分配一个request,再将bio插入***/
/*
* 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);//确定读写标识
if (sync)
rw_flags |= REQ_RW_SYNC;
/*
* Grab a free request. This is might sleep but can not fail.
* Returns with the queue unlocked.
*/
req = get_request_wait(q, rw_flags, bio);//分配一个新的request
/*
* 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.
*/
//根据bio初始化新分配的request，并将bio插入到request中
init_request_from_bio(req, bio);
spin_lock_irq(q->queue_lock);
if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags) ||
bio_flagged(bio, BIO_CPU_AFFINE))
req->cpu = blk_cpu_to_group(smp_processor_id());
if (queue_should_plug(q) && elv_queue_empty(q))
blk_plug_device(q);
add_request(q, req);//将request插入到请求队列
out:
if (unplug || !queue_should_plug(q))
__generic_unplug_device(q);
spin_unlock_irq(q->queue_lock);
return 0;
}
elv_merge()是执行合并的关键所在，执行完后会有三种情况：
1.bio添加到了一个request的bio链表尾部
2.bio添加到了一个request的bio链表首部
3.未能找到一个request可以添加，将重新分配一个request
int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
{
struct elevator_queue *e = q->elevator;
struct request *__rq;
int ret;
/*
* First try one-hit cache.
*/
//last_merge指向了最近进行合并操作的request,最先试图将bio合并到该request中
if (q->last_merge) {
ret = elv_try_merge(q->last_merge, bio);
if (ret != ELEVATOR_NO_MERGE) {
*req = q->last_merge;
return ret;
}
}
if (blk_queue_nomerges(q))//请求队列不允许合并请求，则返回NO_MERGE
return ELEVATOR_NO_MERGE;
/*
* See if our hash lookup can find a potential backmerge.
*/
//根据bio的起始扇区号，通过rq的哈希表寻找一个request,可以将bio合并到request的尾部
__rq = elv_rqhash_find(q, bio->bi_sector);
if (__rq && elv_rq_merge_ok(__rq, bio)) {
*req = __rq;
return ELEVATOR_BACK_MERGE;
}
/*如果以上的方法不成功，则调用特定于io调度器的elevator_merge_fn??5BD数寻找一个合适的request*/
if (e->ops->elevator_merge_fn)
return e->ops->elevator_merge_fn(q, req, bio);
return ELEVATOR_NO_MERGE;
}