Re: [PATCH V11 14/19] block: handle non-cluster bio out of blk_bio_segment_split

[Christoph Hellwig]

On Thu, Nov 22, 2018 at 06:32:09PM +0800, Ming Lei wrote:
> 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.
> > 
> > > What we want to do is just to avoid to merge bvecs to segment, which
> > > should have been done by NO_SG_MERGE simply. However, after multi-page
> > > is enabled, two adjacent bvecs won't be merged any more, I just forget
> > > to remove the bvec merge code in V11.
> > > 
> > > So seems we can simply avoid to use virt boundary limit for non-cluster
> > > after multipage bvec is enabled?
> > 
> > No, we can't just remove it.  As explained in the patch there is one very
> > visible difference of setting the flag amd that is no segment will span a
> > page boundary, and at least the iSCSI code seems to rely on that.
> 
> IMO, we should use queue_segment_boundary() to enhance the rule during splitting
> segment after multi-page bvec is enabled.
> 
> Seems we miss the segment boundary limit in bvec_split_segs().

Yes, that looks like the right fix!