Issues about the merge_bvec_fn callback in 3.10 series

[Jianchao Wang <jianchao.wan9@xxxxxxxxx>]

Hi dear all

This is a question in older kernel versions.

We are using 3.10 series kernel in our production. And we encountered issue as below,

When add a page into a bio, .merge_bvec_fn will be invoked down to the bottom,
and the bio->bi_rw would be saved into bvec_merge_data.bi_rw as the following code,

__bio_add_page
---
	if (q->merge_bvec_fn) {
		struct  bvm = {
			.bi_bdev = bio->bi_bdev,
			.bi_sector = bio->bi_iter.bi_sector,
			.bi_size = bio->bi_iter.bi_size,
			.bi_rw = bio->bi_rw,
		};

		/*
		 * merge_bvec_fn() returns number of bytes it can accept
		 * at this offset
		 */
		if (q->merge_bvec_fn(q, &bvm, bvec) < bvec->bv_len) {
			bvec->bv_page = NULL;
			bvec->bv_len = 0;
			bvec->bv_offset = 0;
			return 0;
		}
	}
---

However, it seems that the bio->bi_rw has not been set at the moment (set by submit_bio), 
so it is always zero.

We have a raid5 and the raid5_mergeable_bvec would always handle the write as read and then
we always get a write bio with a stripe chunk size which is not expected and would degrade the
performance. This is code,

raid5_mergeable_bvec
---
	if ((bvm->bi_rw & 1) == WRITE)
		return biovec->bv_len; /* always allow writes to be mergeable */

	if (mddev->new_chunk_sectors < mddev->chunk_sectors)
		chunk_sectors = mddev->new_chunk_sectors;
	max =  (chunk_sectors - ((sector & (chunk_sectors - 1)) + bio_sectors)) << 9;
	if (max < 0) max = 0;
	if (max <= biovec->bv_len && bio_sectors == 0)
		return biovec->bv_len;
	else
		return max;

---

I have checked   
v3.10.108
v3.18.140
v4.1.49
but there seems not fix for it.

And maybe it would be fixed until 
8ae126660fddbeebb9251a174e6fa45b6ad8f932
block: kill merge_bvec_fn() completely

Would anyone please give some suggestion on this ?
Any comment will be welcomed.

Thanks in advance
Jianchao