Immutable biovecs and biovec iterators:
=======================================

Kent Overstreet <kmo@daterainc.com>

As of 3.13, biovecs should never be modified after a bio has been submitted.
Instead, we have a new struct bvec_iter which represents a range of a biovec -
the iterator will be modified as the bio is completed, not the biovec.

More specifically, old code that needed to partially complete a bio would
update bi_sector and bi_size, and advance bi_idx to the next biovec. If it
ended up partway through a biovec, it would increment bv_offset and decrement
bv_len by the number of bytes completed in that biovec.

In the new scheme of things, everything that must be mutated in order to
partially complete a bio is segregated into struct bvec_iter: bi_sector,
bi_size and bi_idx have been moved there; and instead of modifying bv_offset
and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of
bytes completed in the current bvec.

There are a bunch of new helper macros for hiding the gory details - in
particular, presenting the illusion of partially completed biovecs so that
normal code doesn't have to deal with bi_bvec_done.

 * Driver code should no longer refer to biovecs directly; we now have
   bio_iovec() and bio_iovec_iter() macros that return literal struct biovecs,
   constructed from the raw biovecs but taking into account bi_bvec_done and
   bi_size.

   bio_for_each_segment() has been updated to take a bvec_iter argument
   instead of an integer (that corresponded to bi_idx); for a lot of code the
   conversion just required changing the types of the arguments to
   bio_for_each_segment().

 * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a
   wrapper around bio_advance_iter() that operates on bio->bi_iter, and also
   advances the bio integrity's iter if present.

   There is a lower level advance function - bvec_iter_advance() - which takes
   a pointer to a biovec, not a bio; this is used by the bio integrity code.

What's all this get us?
=======================

Having a real iterator, and making biovecs immutable, has a number of
advantages:

 * Before, iterating over bios was very awkward when you weren't processing
   exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c,
   which copies the contents of one bio into another. Because the biovecs
   wouldn't necessarily be the same size, the old code was tricky convoluted -
   it had to walk two different bios at the same time, keeping both bi_idx and
   and offset into the current biovec for each.

   The new code is much more straightforward - have a look. This sort of
   pattern comes up in a lot of places; a lot of drivers were essentially open
   coding bvec iterators before, and having common implementation considerably
   simplifies a lot of code.

 * Before, any code that might need to use the biovec after the bio had been
   completed (perhaps to copy the data somewhere else, or perhaps to resubmit
   it somewhere else if there was an error) had to save the entire bvec array
   - again, this was being done in a fair number of places.

 * Biovecs can be shared between multiple bios - a bvec iter can represent an
   arbitrary range of an existing biovec, both starting and ending midway
   through biovecs. This is what enables efficient splitting of arbitrary
   bios. Note that this means we _only_ use bi_size to determine when we've
   reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes
   bi_size into account when constructing biovecs.

 * Splitting bios is now much simpler. The old bio_split() didn't even work on
   bios with more than a single bvec! Now, we can efficiently split arbitrary
   size bios - because the new bio can share the old bio's biovec.

   Care must be taken to ensure the biovec isn't freed while the split bio is
   still using it, in case the original bio completes first, though. Using
   bio_chain() when splitting bios helps with this.

 * Submitting partially completed bios is now perfectly fine - this comes up
   occasionally in stacking block drivers and various code (e.g. md and
   bcache) had some ugly workarounds for this.

   It used to be the case that submitting a partially completed bio would work
   fine to _most_ devices, but since accessing the raw bvec array was the
   norm, not all drivers would respect bi_idx and those would break. Now,
   since all drivers _must_ go through the bvec iterator - and have been
   audited to make sure they are - submitting partially completed bios is
   perfectly fine.

Other implications:
===================

 * Almost all usage of bi_idx is now incorrect and has been removed; instead,
   where previously you would have used bi_idx you'd now use a bvec_iter,
   probably passing it to one of the helper macros.

   I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you
   now use bio_iter_iovec(), which takes a bvec_iter and returns a
   literal struct bio_vec - constructed on the fly from the raw biovec but
   taking into account bi_bvec_done (and bi_size).

 * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that
   doesn't actually own the bio. The reason is twofold: firstly, it's not
   actually needed for iterating over the bio anymore - we only use bi_size.
   Secondly, when cloning a bio and reusing (a portion of) the original bio's
   biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate
   over all the biovecs in the new bio - which is silly as it's not needed.

   So, don't use bi_vcnt anymore.