[RFC PATCH v4 00/20] mm/block: add bdi sysfs knobs

[Stefan Roesch <shr@devkernel.io>]

At meta network block devices (nbd) are used to implement remote block
storage. In testing and during production it has been observed that
these network block devices can consume a huge portion of the dirty
writeback cache and writeback can take a considerable time.

To be able to give stricter limits, I'm proposing the following changes:

1) introduce strictlimit knob

  Currently the max_ratio knob exists to limit the dirty_memory. However
  this knob only applies once (dirty_ratio + dirty_background_ratio) / 2
  has been reached.
  With the BDI_CAP_STRICTLIMIT flag, the max_ratio can be applied without
  reaching that limit. This change exposes that knob.

  This knob can also be useful for NFS, fuse filesystems and USB devices.

2) Use part of 1000000 internal calculation

  The max_ratio is based on percentage. With the current machine sizes
  percentage values can be very high (1% of a 256GB main memory is already
  2.5GB). This change uses part of 1000000 instead of percentages for the
  internal calculations.

3) Introduce two new sysfs knobs: min_bytes and max_bytes.

  Currently all calculations are based on ratio, but for a user it often
  more convenient to specify a limit in bytes. The new knobs will not
  store bytes values, instead they will translate the byte value to a
  corresponding ratio. As the internal values are now part of 1000, the
  ratio is closer to the specified value. However the value should be more
  seen as an approximation as it can fluctuate over time.


3) Introduce two new sysfs knobs: min_ratio_fine and max_ratio_fine.

  The granularity for the existing sysfs bdi knobs min_ratio and max_ratio
  is based on percentage values. The new sysfs bdi knobs min_ratio_fine
  and max_ratio_fine allow to specify the ratio as part of 1 million.

Changes:
  V4:
  - Introduced two new sysfs knobs min_ratio_fine and max_ratio_fine to allow
    setting ratios with smaller granularity
  - Refreshed to 6.1-rc5
  - removed bdi_set_strict_limit export
  - removed bdi_get_max_bytes export
  - removed bdi_set_max_bytes export
  - change granularity to part of 1000000
  - changed function signature of bdi_get_max_bytes() to return u64
  - Fixed commit message of
    "mm: split off __bdi_set_max_ratio() function"
  - changed check in bdi_check_pages_limit()
  V3:
  - change signature of function bdi_ratio_from_pages to take an unsigned long
    parameter
  - use div64_u64 function for division to support 32 bit platforms
  - Refreshed to 6.1-rc2

  V2:
  - Refreshed to 6.1-rc1
  - Use part of 1000, instead of part of 10000
  - Reformat cover letter


Stefan Roesch (20):
  mm: add bdi_set_strict_limit() function
  mm: add knob /sys/class/bdi/<bdi>/strict_limit
  mm: document /sys/class/bdi/<bdi>/strict_limit knob
  mm: use part per 1000000 for bdi ratios.
  mm: add bdi_get_max_bytes() function
  mm: split off __bdi_set_max_ratio() function
  mm: add bdi_set_max_bytes() function.
  mm: add knob /sys/class/bdi/<bdi>/max_bytes
  mm: document /sys/class/bdi/<bdi>/max_bytes knob
  mm: add bdi_get_min_bytes() function.
  mm: split off __bdi_set_min_ratio() function
  mm: add bdi_set_min_bytes() function
  mm: add /sys/class/bdi/<bdi>/min_bytes knob
  mm: document /sys/class/bdi/<bdi>/min_bytes knob
  mm: add bdi_set_max_ratio_no_scale() function
  mm: add /sys/class/bdi/<bdi>/max_ratio_fine knob
  mm: document /sys/class/bdi/<bdi>/max_ratio_fine knob
  mm: add bdi_set_min_ratio_no_scale() function
  mm: add /sys/class/bdi/<bdi>/min_ratio_fine knob
  mm: document /sys/class/bdi/<bdi>/min_ratio_fine knob

 Documentation/ABI/testing/sysfs-class-bdi |  68 +++++++++++
 include/linux/backing-dev.h               |  10 ++
 mm/backing-dev.c                          | 133 +++++++++++++++++++++-
 mm/page-writeback.c                       | 130 +++++++++++++++++++--
 4 files changed, 329 insertions(+), 12 deletions(-)


base-commit: ab290eaddc4c41b237b9a366fa6a5527be890b84
-- 
2.30.2