On Thu, Sep 20, 2012 at 05:27:06PM -0700, Tejun Heo wrote:

On Mon, Sep 10, 2012 at 05:22:30PM -0700, Kent Overstreet wrote:

quoted

A bunch of what __blk_queue_bounce() was doing was problematic for the
immutable bvec work; this cleans that up and the code is quite a bit
smaller, too.

The __bio_for_each_segment() in copy_to_high_bio_irq() was changed
because that one's looping over the original bio, not the bounce bio -
since the bounce code doesn't own that bio the __ version wasn't
correct.

Also, I can't understand the above at all.  I can think why it
wouldn't be necessary but why is it wrong because bounce code doesn't
own it?

Another prep work thing - in current code, it isn't really wrong
(slightly inconsistent though).

But the idea is that anything that doesn't own the bio shouldn't assume
anything about bi_idx; the bounce code should loop over the bio starting
from wherever it was when the bio got to the bounce code, not the start
of the bio.

A later patch makes this clearer - __bio_for_each_segment() gets removed
in favor of bio_for_each_segment_all(), and it documents that
bio_for_each_segment_all() is only for code that owns the bio.