[PATCH v1 0/4] Allow order zero pages in page reporting

[Yuvraj Sakshith]

Today, page reporting sets page_reporting_order in two ways:

(1) page_reporting.page_reporting_order cmdline parameter
(2) Driver can pass order while registering itself.

In both cases, order zero is ignored by free page reporting
because it is used to set page_reporting_order to a default
value, like MAX_PAGE_ORDER.

In some cases we might want page_reporting_order to be zero.

For instance, when virtio-balloon runs inside a guest with
tiny memory (say, 16MB), it might not be able to find a order 1 page
(or in the worst case order MAX_PAGE_ORDER page) after some uptime.
Page reporting should be able to return order zero pages back for
optimal memory relinquishment.

This patch changes the default fallback value from '0' to '-1' in
all possible clients of free page reporting (hv_balloon and
virtio-balloon) together with allowing '0' as a valid order in
page_reporting_register().

Changes in v1:
- Introduce PAGE_REPORTING_DEFAULT_ORDER macro (initially set to 0).
- Make use of new macro in drivers (hv_balloon and virtio-balloon)
	working with page reporting.
- Change PAGE_REPORTING_DEFAULT_ORDER to -1 as zero is a valid
	page order that can be requested.

Yuvraj Sakshith (3):
  mm/page_reporting: Allow zero page_reporting_order
  hv_balloon: Change default page reporting order
  virtio_balloon: Set pr_dev.order to new default

 drivers/hv/hv_balloon.c         |  2 +-
 drivers/virtio/virtio_balloon.c | 14 ++++++++++++++
 mm/page_reporting.c             |  2 +-
 3 files changed, 16 insertions(+), 2 deletions(-)

-- 
2.34.1