If your joists are at least 7.25" deep or more (the nominal depth of a milled 2x8) it'll fit pretty nicely. If the joists are full-dimension rough 2x lumber, shallower than 7.25", or the spacing isn't a standard 16" or 24" on center spacing you'll be better off using blown cellulose, installed at an initial depth of 9.5-10", with purpose made ventilation chutes to prevent it from contacting the roof deck (which would interfere with the drying capacity of the roof deck.) Cellulose is a pretty cheap DIY, and box stores usually throw in a day of free blower rental with some minimum quantity purchase of cellulose.
Rock wool is good stuff though- just be sure to trim it carefully to fit with no gaps or compressions . Use a purpose-made batt knife or an 8" (or longer) bread knife for trimming. Rock wool can get pretty messed up if trying to cut it with a utility knife, more so than with low-density fiberglass (which should also be trimmed with a batt knife. DIYers often try get by with a utility knife, which can sort-of work with fiberglass, but the fit and mess can be pretty lousy when using the wrong tool. A batt knife isn't really very expensive, and makes the job a lot easier.
Prior to insulating take pains to fully air seal the ceiling/attic-floor around all electrical, plumbing, & flue penetrations with the appropriate materials.