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: > > 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