Linux Block块设备层内存映射

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参考

Linux那些事儿之我是Block层(11)传说中的内存映射(上)
Linux那些事儿之我是Block层(12)传说中的内存映射(下)
linux内核分析：read过程分析
write每次最大能写多少字节的数据
The iov_iter interface

用户态内存的映射

Linux使用blk_rq_map_user来完成用户态内存和bio的映射，blk_rq_map_user会填充struct request的struct bio结构体，

int blk_rq_map_user(struct request_queue *q, struct request *rq,
		    struct rq_map_data *map_data, void __user *ubuf,
		    unsigned long len, gfp_t gfp_mask)
{
	struct iovec iov;
	struct iov_iter i;
	int ret = import_single_range(rq_data_dir(rq), ubuf, len, &iov, &i);

	if (unlikely(ret < 0))
		return ret;

	return blk_rq_map_user_iov(q, rq, map_data, &i, gfp_mask);
}
EXPORT_SYMBOL(blk_rq_map_user);

函数import_single_range中，MAX_RW_COUNT是一个宏：INT_MAX & PAGE_MASK，INT_MAX是2^31，理论上每次write可写的buff大小是2^31-2^12=2147479552，然后使用了迭代器，

int import_single_range(int rw, void __user *buf, size_t len,
		 struct iovec *iov, struct iov_iter *i)
{
	if (len > MAX_RW_COUNT)
		len = MAX_RW_COUNT;
	if (unlikely(!access_ok(!rw, buf, len)))
		return -EFAULT;

	iov->iov_base = buf;
	iov->iov_len = len;
	iov_iter_init(i, rw, iov, 1, len);
	return 0;
}
EXPORT_SYMBOL(import_single_range);

blk_rq_map_user_iov调用__blk_rq_map_user_iov

/**
 * blk_rq_map_user_iov - map user data to a request, for passthrough requests
 * @q:		request queue where request should be inserted
 * @rq:		request to map data to
 * @map_data:   pointer to the rq_map_data holding pages (if necessary)
 * @iter:	iovec iterator
 * @gfp_mask:	memory allocation flags
 *
 * Description:
 *    Data will be mapped directly for zero copy I/O, if possible. Otherwise
 *    a kernel bounce buffer is used.
 *
 *    A matching blk_rq_unmap_user() must be issued at the end of I/O, while
 *    still in process context.
 *
 *    Note: The mapped bio may need to be bounced through blk_queue_bounce()
 *    before being submitted to the device, as pages mapped may be out of
 *    reach. It's the callers responsibility to make sure this happens. The
 *    original bio must be passed back in to blk_rq_unmap_user() for proper
 *    unmapping.
 */
int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
			struct rq_map_data *map_data,
			const struct iov_iter *iter, gfp_t gfp_mask)
{
	bool copy = false;
	unsigned long align = q->dma_pad_mask | queue_dma_alignment(q);
	struct bio *bio = NULL;
	struct iov_iter i;
	int ret;

	if (!iter_is_iovec(iter))
		goto fail;

	if (map_data)
		copy = true;
	else if (iov_iter_alignment(iter) & align)
		copy = true;
	else if (queue_virt_boundary(q))
		copy = queue_virt_boundary(q) & iov_iter_gap_alignment(iter);

	i = *iter;
	do {
		ret =__blk_rq_map_user_iov(rq, map_data, &i, gfp_mask, copy);
		if (ret)
			goto unmap_rq;
		if (!bio)
			bio = rq->bio;
	} while (iov_iter_count(&i));

	if (!bio_flagged(bio, BIO_USER_MAPPED))
		rq->rq_flags |= RQF_COPY_USER;
	return 0;

unmap_rq:
	__blk_rq_unmap_user(bio);
fail:
	rq->bio = NULL;
	return -EINVAL;
}
EXPORT_SYMBOL(blk_rq_map_user_iov);

函数bio_add_pc_page把page添加到bio，

/**
 *	bio_add_pc_page	-	attempt to add page to bio
 *	@q: the target queue
 *	@bio: destination bio
 *	@page: page to add
 *	@len: vec entry length
 *	@offset: vec entry offset
 *
 *	Attempt to add a page to the bio_vec maplist. This can fail for a
 *	number of reasons, such as the bio being full or target block device
 *	limitations. The target block device must allow bio's up to PAGE_SIZE,
 *	so it is always possible to add a single page to an empty bio.
 *
 *	This should only be used by REQ_PC bios.
 */
int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
		    *page, unsigned int len, unsigned int offset)
{
	int retried_segments = 0;
	struct bio_vec *bvec;

	/*
	 * cloned bio must not modify vec list
	 */
	if (unlikely(bio_flagged(bio, BIO_CLONED)))
		return 0;

	if (((bio->bi_iter.bi_size + len) >> 9) > queue_max_hw_sectors(q))
		return 0;

	/*
	 * For filesystems with a blocksize smaller than the pagesize
	 * we will often be called with the same page as last time and
	 * a consecutive offset.  Optimize this special case.
	 */
	if (bio->bi_vcnt > 0) {
		struct bio_vec *prev = &bio->bi_io_vec[bio->bi_vcnt - 1];

		if (page == prev->bv_page &&
		    offset == prev->bv_offset + prev->bv_len) {
			prev->bv_len += len;
			bio->bi_iter.bi_size += len;
			goto done;
		}

		/*
		 * If the queue doesn't support SG gaps and adding this
		 * offset would create a gap, disallow it.
		 */
		if (bvec_gap_to_prev(q, prev, offset))
			return 0;
	}

	if (bio->bi_vcnt >= bio->bi_max_vecs)
		return 0;

	/*
	 * setup the new entry, we might clear it again later if we
	 * cannot add the page
	 */
	bvec = &bio->bi_io_vec[bio->bi_vcnt];
	bvec->bv_page = page;
	bvec->bv_len = len;
	bvec->bv_offset = offset;
	bio->bi_vcnt++;
	bio->bi_phys_segments++;
	bio->bi_iter.bi_size += len;

	/*
	 * Perform a recount if the number of segments is greater
	 * than queue_max_segments(q).
	 */

	while (bio->bi_phys_segments > queue_max_segments(q)) {

		if (retried_segments)
			goto failed;

		retried_segments = 1;
		blk_recount_segments(q, bio);
	}

	/* If we may be able to merge these biovecs, force a recount */
	if (bio->bi_vcnt > 1 && (BIOVEC_PHYS_MERGEABLE(bvec-1, bvec)))
		bio_clear_flag(bio, BIO_SEG_VALID);

 done:
	return len;

 failed:
	bvec->bv_page = NULL;
	bvec->bv_len = 0;
	bvec->bv_offset = 0;
	bio->bi_vcnt--;
	bio->bi_iter.bi_size -= len;
	blk_recount_segments(q, bio);
	return 0;
}
EXPORT_SYMBOL(bio_add_pc_page);

request映射

使用blk_rq_map_sg把request映射到一个scatterlist，

//petalinux2015.2.1, kernel 3.19
blk_rq_map_sg //block\blk-merge.c
  __blk_bios_map_sg
    __blk_segment_map_sg

在__blk_segment_map_sg中，BIOVEC_PHYS_MERGEABLE检测相邻bio_vec是否可以合并，BIOVEC_SEG_BOUNDARY用于检测是否超过边界，比如某些硬件不能跨64MB边界传输，其单次发起DMA的最大大小就是64MB，可以通过queue_max_segment_size来控制scatterlist的表项。

static inline void
__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
		     struct scatterlist *sglist, struct bio_vec *bvprv,
		     struct scatterlist **sg, int *nsegs, int *cluster)
{

	int nbytes = bvec->bv_len;

	if (*sg && *cluster) {
		if ((*sg)->length + nbytes > queue_max_segment_size(q))
			goto new_segment;

		if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
			goto new_segment;
		if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
			goto new_segment;

		(*sg)->length += nbytes;
	} else {
new_segment:
		if (!*sg)
			*sg = sglist;
		else {
			/*
			 * If the driver previously mapped a shorter
			 * list, we could see a termination bit
			 * prematurely unless it fully inits the sg
			 * table on each mapping. We KNOW that there
			 * must be more entries here or the driver
			 * would be buggy, so force clear the
			 * termination bit to avoid doing a full
			 * sg_init_table() in drivers for each command.
			 */
			sg_unmark_end(*sg);
			*sg = sg_next(*sg);
		}

		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
		(*nsegs)++;
	}
	*bvprv = *bvec;
}