xenbus/backend: Add a memory pressure handler callback

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lack
flexibility.

To mitigate such problems, this patchset adds a memory reclaim callback
to 'xenbus_driver' (patch 1) and use it to mitigate the problem in
'xen-blkback' (patch 2).  The third and fourth patches are trivial
cleanups.

Base Version
------------

This patch is based on v5.4.  A complete tree is also available at my
public git repo:
https://github.com/sjp38/linux/tree/blkback_squeezing_v10


Patch History
-------------

Changes from v9
(https://lore.kernel.org/xen-devel/20191213153546.17425-1-...)
 - Add 'Reviewed-by' and 'Acked-by' from Roger Pau Monné
 - Update the commit message for overhead test of the 2nd path

Changes from v8
(https://lore.kernel.org/xen-devel/20191213130211.24011-1-...)
 - Drop 'Reviewed-by: Juergen' from the second patch
   (suggested by Roger Pau Monné)
 - Update contact of the new module param to SeongJae Park
   <sjpark@amazon.de>
   (suggested by Roger Pau Monné)
 - Wordsmith the description of the parameter
   (suggested by Roger Pau Monné)
 - Fix dumb bugs
   (suggested by Roger Pau Monné)
 - Move module param definition to xenbus.c and reduce the number of
   lines for this change
   (suggested by Roger Pau Monné)
 - Add a comment for the new callback, reclaim_memory, as other
   callbacks also have
 - Add another trivial cleanup of xenbus.c file (4th patch)

Changes from v7
(https://lore.kernel.org/xen-devel/20191211181016.14366-1-...)
 - Update sysfs-driver-xen-blkback for new parameter
   (suggested by Roger Pau Monné)
 - Use per-xen_blkif buffer_squeeze_end instead of global variable
   (suggested by Roger Pau Monné)

Changes from v6
(https://lore.kernel.org/linux-block/20191211042428.5961-1...)
 - Remove more unnecessary prefixes (suggested by Roger Pau Monné)
 - Constify a variable (suggested by Roger Pau Monné)
 - Rename 'reclaim' into 'reclaim_memory' (suggested by Roger Pau Monné)
 - More wordsmith of the commit message (suggested by Roger Pau Monné)

Changes from v5
(https://lore.kernel.org/linux-block/20191210080628.5264-1...)
 - Wordsmith the commit messages (suggested by Roger Pau Monné)
 - Change the reclaim callback return type (suggested by Roger Pau
   Monné)
 - Change the type of the blkback squeeze duration variable
   (suggested by Roger Pau Monné)
 - Add a patch for removal of unnecessary static variable name prefixes
   (suggested by Roger Pau Monné)
 - Fix checkpatch.pl warnings

Changes from v4
(https://lore.kernel.org/xen-devel/20191209194305.20828-1-...)
 - Remove domain id parameter from the callback (suggested by Juergen
   Gross)
 - Rename xen-blkback module parameter (suggested by Stefan Nuernburger)

Changes from v3
(https://lore.kernel.org/xen-devel/20191209085839.21215-1-...)
 - Add general callback in xen_driver and use it (suggested by Juergen
   Gross)

Changes from v2
(https://lore.kernel.org/linux-block/af195033-23d5-38ed-b7...)
 - Rename the module parameter and variables for brevity
   (aggressive shrinking -> squeezing)

Changes from v1
(https://lore.kernel.org/xen-devel/20191204113419.2298-1-s...)
 - Adjust the description to not use the term, `arbitrarily`
   (suggested by Paul Durrant)
 - Specify time unit of the duration in the parameter description,
   (suggested by Maximilian Heyne)
 - Change default aggressive shrinking duration from 1ms to 10ms
 - Merge two patches into one single patch

SeongJae Park (4):
  xenbus/backend: Add memory pressure handler callback
  xen/blkback: Squeeze page pools if a memory pressure is detected
  xen/blkback: Remove unnecessary static variable name prefixes
  xen/blkback: Consistently insert one empty line between functions

 .../ABI/testing/sysfs-driver-xen-blkback      | 10 +++++
 drivers/block/xen-blkback/blkback.c           | 42 +++++++++----------
 drivers/block/xen-blkback/common.h            |  1 +
 drivers/block/xen-blkback/xenbus.c            | 26 +++++++++---
 drivers/xen/xenbus/xenbus_probe_backend.c     | 32 ++++++++++++++
 include/xen/xenbus.h                          |  1 +
 6 files changed, 86 insertions(+), 26 deletions(-)

-- 
2.17.1