On Tue, Mar 07, 2023 at 09:24:31PM +0800, Chuang Xu wrote: > > Why do we need address_space_get_flatview_rcu()? I'm not sure whether you > > address_space_cahce_init() uses address_space_get_flatview() to acquire > a ref-ed flatview. If we want to use address_space_to_flatview_rcu() and > make the flatview ref-ed, maybe we need to add > address_space_get_flatview_rcu()? > Or if we use address_space_to_flatview_rcu() directly, then we'll get a > unref-ed flatview, I'm not sure wheather it's safe in other cases.. At least > I don't want the assertion to be triggered one day.
No we can't touch that, afaict. It was a real fix (447b0d0b9e) to a real bug. What I meant is we make address_space_get_flatview() always use address_space_to_flatview_rcu(). I think it's safe, if you see the current callers of it (after my patch 1 fixed version applied): memory_region_sync_dirty_bitmap[2249] view = address_space_get_flatview(as); memory_region_update_coalesced_range[2457] view = address_space_get_flatview(as); memory_region_clear_dirty_bitmap[2285] view = address_space_get_flatview(as); mtree_info_flatview[3418] view = address_space_get_flatview(as); address_space_cache_init[3350] cache->fv = address_space_get_flatview(as); Case 5 is what we're targeting for which I think it's fine. Logically any commit() hook should just use address_space_get_flatview_raw() to reference the flatview, but at least address_space_cache_init() is also called in non-BQL sections, so _rcu is the only thing we can use here, iiuc.. Case 1-3 is never called during vm load, so I think it's safe. Case 4 can be accessing stalled flatview but I assume fine since it's just for debugging. E.g. if someone tries "info mtree -f" during vm loading after your patch applied, then one can see stalled info. I don't think it can even happen because so far "info mtree" should also take BQL.. so it'll be blocked until vm load completes. The whole thing is still tricky. I didn't yet make my mind through on how to make it very clean, I think it's slightly beyond what this series does and I need some more thinkings (or suggestions from others). -- Peter Xu
