RFC mm/block: add bdi sysfs knobs

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