dm vdo: cleanup style for comments in structs

Use /* ... */ rather than /** ... */ if for no other reason than
syntax highlighting is improved (at least for me, in emacs: comments
are now red, code is yellow. Previously comments were also yellow).

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Susan LeGendre-McGhee <slegendr@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>

dm vdo: add documentation details on zones and locking

Add details describing the vdo zone and thread model to the
documentation comments for major vdo components. Also added
some high-level description of the block map structure.

Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

dm vdo block-map: rename struct cursors member to 'completion'

'completion' is more informative name for a 'struct vdo_completion'
than 'parent'.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>

dm vdo wait-queue: add proper namespace to interface

Rename various interfaces and structs associated with vdo's wait-queue,
e.g.: s/wait_queue/vdo_wait_queue/, s/waiter/vdo_waiter/, etc.

Now all function names start with "vdo_waitq_" or "vdo_waiter_".

Reviewed-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>

dm vdo: implement the block map page cache

The set of leaf pages of the block map tree is too large to fit in memory,
so each block map zone maintains a cache of leaf pages. This patch adds the
implementation of that cache.

Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
Co-developed-by: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Michael Sclafani <dm-devel@lists.linux.dev>
Co-developed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

dm vdo: add the block map

The block map contains the logical to physical mapping. It can be thought
of as an array with one entry per logical address. Each entry is 5 bytes:
36 bits contain the physical block number which holds the data for the
given logical address, and the remaining 4 bits are used to indicate the
nature of the mapping. Of the 16 possible states, one represents a logical
address which is unmapped (i.e. it has never been written, or has been
discarded), one represents an uncompressed block, and the other 14 states
are used to indicate that the mapped data is compressed, and which of the
compression slots in the compressed block this logical address maps to.

Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
Co-developed-by: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Michael Sclafani <dm-devel@lists.linux.dev>
Co-developed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>