On Fri, Feb 10, 2023 at 12:29:54PM +0100, Jan Kara wrote:

functionally that would make sense but as I've mentioned in my reply to you
[1], the problem here is the performance. I've now dug out the discussion
from 2018 where John actually tried to take pinned pages out of the LRU [2]
and the result was 20% IOPS degradation on his NVME drive because of the
cost of taking the LRU lock. I'm not even speaking how costly that would
get on any heavily parallel direct IO workload on some high-iops device...

I think we need to distinguish between short- and long terms pins.
For short term pins like direct I/O it doesn't make sense to take them
off the lru, or to do any other special action.  Writeback will simplify
have to wait for the short term pin.

Long-term pins absolutely would make sense to be taken off the LRU list.