The remembered bit is stored in the JSCell, and that's where everyone looks for it. We were keeping an atomic bitmap up-to-date in the MarkedBlock for no reason.
Created attachment 253463 [details] Patch
Created attachment 253464 [details] Patch
Comment on attachment 253464 [details] Patch Whoops :-) r=me
Landed in <https://trac.webkit.org/r184666>