Evicting too many bytes from MFU metadata

[tkittich]

Motivation and Context

I am not so sure about this PR. Consider this as a small bug report. ^^"
From reading the code, it seems some bytes has been evicted from MRU metadata so it probably should be subtracted from m.

How Has This Been Tested?

Has not been tested.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

I think you are right here. My fault. Without updating m there we evict from MFU metadata all that we wanted to evict from all metadata, including already evicted MRU metadata (m is the total amount of metadata we had at the beginning, and w is the total amount of metadata we want to have).

Since you already have reproduction for the issue, could you test that it behaves better?

[behlendorf]

Good find, yes I believe you're right. If you could provide some of your test results that would be very helpful.