Re: [PATCH v2 19/26] bounce: Refactor __blk_queue_bounce to not use bi_io_vec

[Kent Overstreet <koverstreet@google.com>]

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:
> > 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.