On Thu, Nov 22, 2018 at 11:04:28AM +0100, Christoph Hellwig wrote:

On Thu, Nov 22, 2018 at 05:33:00PM +0800, Ming Lei wrote:

quoted

However, using virt boundary limit on non-cluster seems over-kill,
because the bio will be over-split(each small bvec may be split as one bio)
if it includes lots of small segment.

The combination of the virt boundary of PAGE_SIZE - 1 and a
max_segment_size of PAGE_SIZE will only split if the to me merged
segment is in a different page than the previous one, which is exactly
what we need here.  Multiple small bvec inside the same page (e.g.
512 byte buffer_heads) will still be merged.

Suppose one bio includes (pg0, 0, 512) and (pg1, 512, 512):

The split is introduced by the following code in blk_bio_segment_split():

      if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
		  	goto split;

Without this patch, for non-cluster, the two bvecs are just in different
segment, but still handled by one same bio. Now you convert into two bios.

Thanks,
Ming