What were you trying to do that didn't work?

Switch between profiles where vm.dirty_bytes or vm.dirty_background_bytes are being used. For more detail, please see the reproducer.

What is the impact of this issue to you?

Unability to roll-back settings during profile switches.

Please provide the package NVR for which the bug is seen:

tuned-2.22.1-1.4.20240530git5385fa99.el9fdp.noarch

How reproducible is this bug?:

Always.

Steps to reproduce

1. mkdir -p /etc/tuned/p {1,2}
2. cat > /etc/tuned/p1/tuned.conf <<EOF
   [main]
   summary=Empty profile
   EOF
3. cat > /etc/tuned/p2/tuned.conf <<EOF
   [main]
   summary=Customize dirty memory
   [sysctl]
   vm.dirty_bytes = 8192
   vm.dirty_background_bytes = 8192
   EOF
4. tuned-adm profile p1
5. reboot # reboot to get the default kernel settings and set empty TuneD p1 profile
6. sysctl -a|grep -E 'vm.dirty_(bytes|ratio)'
   vm.dirty_bytes = 0
   vm.dirty_ratio = 20
7. tuned-adm active
   Current active profile: p1
8. tuned-adm profile p2
9. tuned-adm profile p1
10. sysctl -a|grep -E 'vm.dirty_(bytes|ratio)'
    vm.dirty_bytes = 8192
    vm.dirty_ratio = 0
11. # Note the sysctls values did not roll back and see the errors in /var/log/tuned/tuned.log
    grep ERROR /var/log/tuned/tuned.log
    2024-09-13 03:08:37,474 ERROR tuned.utils.commands: Writing to file '/proc/sys/vm/dirty_bytes' error: '[Errno 22] Invalid argument'
    2024-09-13 03:08:37,474 ERROR tuned.utils.commands: Writing to file '/proc/sys/vm/dirty_background_bytes' error: '[Errno 22] Invalid argument'

Expected results

vm.dirty_bytes
vm.dirty_ratio
vm.dirty_background_bytes
vm.dirty_background_ratio

values rolled back and no ERROR messages in TuneD logs

Actual results

See the reproducer

Additional information

The issue is that during the rollback, TuneD tries to write 0 to vm.dirty_bytes and vm.dirty_background_bytes. This is not allowed. The miminum value is 2*page size bytes. This can likely be solved by adding additional logic to see what the value of vm.dirty_ratio and vm.dirty_background_ratio is and write that instead when vm.dirty_bytes or vm.dirty_background_bytes are 0s. Writing any value to the _ratio knobs zeros the respective _bytes knobs. See the kernel docs for more detail:

dirty_bytes
===========

Contains the amount of dirty memory at which a process generating disk writes
will itself start writeback.

Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
specified at a time. When one sysctl is written it is immediately taken into
account to evaluate the dirty memory limits and the other appears as 0 when
read.

Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
value lower than this limit will be ignored and the old configuration will be
retained.


dirty_ratio
===========

Contains, as a percentage of total available memory that contains free pages
and reclaimable pages, the number of pages at which a process which is
generating disk writes will itself start writing out dirty data.

The total available memory is not equal to total system memory.