Runaway SLAB usage by 'bio' during 'device replace'

I have a RAID6 array that had a failed HDD. The drive failed completely and has been removed from the system. I'm running a 'device replace' operation with a new disk. The array is ~20TB so this will take a few days. Yesterday the system crashed hard with OOM errors about 24 hours into the

Functional difference between "replace" vs "add" then "delete missing" with a missing disk in a RAID56 array

Situation: A six disk RAID5/6 array with a completely failed disk. The failed disk is removed and an identical replacement drive is plugged in. Here I have two options for replacing the disk, assuming the old drive is device 6 in the superblock and the replacement disk is /dev/sda. 'btrfs