Comparing mempool_create_kmalloc_pool() and mempool_create(), the former requires the pool element size as a parameter, while the latter uses r1bio_pool_alloc() to allocate new elements, with the size calculated based on poolinfo->raid_disks.
The key point is poolinfo, which is used for both r1bio_pool and r1buf_pool.
If we change from mempool_create() to mempool_create_kmalloc_pool(), we would need to introduce a new concept, such as r1bio_pool_size, and store it somewhere. In this case, the original conf->poolinfo would lose its meaning and become just r1buf_poolinfo.
So I think keeping poolinfo is a better fit for the pool in RAID1.