[PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory pressure

[SeongJae Park]

Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and be increased on demand while processing
the I/O requests.  If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.

Therefore, `blkfront` running guests can cause a memory pressure in the
`blkback` running guest by attaching a large number of block devices and
inducing I/O.  System administrators can avoid such problematic
situations by limiting the maximum number of devices each guest can
attach.  However, finding the optimal limit is not so easy.  Improper
set of the limit can results in the memory pressure or a resource
underutilization.  This commit avoids such problematic situations by
squeezing the pools (returns every free page in the pool to the system)
for a while (users can set this duration via a module parameter) if a
memory pressure is detected.


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_aggressive_shrinking_v3


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

Changes from v2 (https://lore.kernel.org/linux-block/af195033-23d5-38ed-b73b-f6e2e3b34541@xxxxxxxxxx)
 - Rename the module parameter and variables for brevity (aggressive
   shrinking -> squeezing)

Changes from v1 (https://lore.kernel.org/xen-devel/20191204113419.2298-1-sjpark@xxxxxxxxxx/)
 - 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 (1):
  xen/blkback: Squeeze page pools if a memory pressure is detected

 drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

-- 
2.17.1