Hi Catalin, I've got a few reports of this over the weekend, but it still doesn't make much sense to me.
Could it be that kmemleak can't deal with the bio_batch logic? I've tried to look at the various bio and biovec number entries in /proc/slabinfo, and while they keep changing a bit during the system runtime there doesn't seem to be a persistent increase even after lots of mkfs calls. Can all of you who have reported the issue take a look at their slabinfo files and check if you can confirm that observation?
