On Tue, Dec 17, 2024 at 08:27:46AM -0800, Suren Baghdasaryan wrote: > > So I just replied there, and no, I don't think it makes sense. Just put > > the kmem_cache_free() in vma_refcount_put(), to be done on 0. > > That's very appealing indeed and makes things much simpler. The > problem I see with that is the case when we detach a vma from the tree > to isolate it, then do some cleanup and only then free it. That's done > in vms_gather_munmap_vmas() here: > https://elixir.bootlin.com/linux/v6.12.5/source/mm/vma.c#L1240 and we > even might reattach detached vmas back: > https://elixir.bootlin.com/linux/v6.12.5/source/mm/vma.c#L1312. IOW, > detached state is not final and we can't destroy the object that > reached this state. Urgh, so that's the munmap() path, but arguably when that fails, the map stays in place. I think this means you're marking detached too soon; you should only mark detached once you reach the point of no return. That said, once you've reached the point of no return; and are about to go remove the page-tables, you very much want to ensure a lack of concurrency. So perhaps waiting for out-standing readers at this point isn't crazy. Also, I'm having a very hard time reading this maple tree stuff :/ Afaict vms_gather_munmap_vmas() only adds the VMAs to be removed to a second tree, it does not in fact unlink them from the mm yet. AFAICT it's vma_iter_clear_gfp() that actually wipes the vmas from the mm -- and that being able to fail is mind boggling and I suppose is what gives rise to much of this insanity :/ Anyway, I would expect remove_vma() to be the one that marks it detached (it's already unreachable through vma_lookup() at this point) and there you should wait for concurrent readers to bugger off. > We could change states to: 0=unused (we can free > the object), 1=detached, 2=attached, etc. but then vma_start_read() > should do something like refcount_inc_more_than_one() instead of > refcount_inc_not_zero(). Would you be ok with such an approach? Urgh, I would strongly suggest ditching refcount_t if we go this route. The thing is; refcount_t should remain a 'simple' straight forward interface and not allow people to do the wrong thing. Its not meant to be the kitchen sink -- we have atomic_t for that. Anyway, the more common scheme at that point is using -1 for 'free', I think folio->_mapcount uses that even. For that see: atomic_add_negative*(). > > Additionally, having vma_end_write() would allow you to put a lockdep > > annotation in vma_{start,end}_write() -- which was I think the original > > reason I proposed it a while back, that and having improved clarity when > > reading the code, since explicitly marking the end of a section is > > helpful. > > The vma->vmlock_dep_map is tracking vma->vm_refcnt, not the > vma->vm_lock_seq (similar to how today vma->vm_lock has its lockdep > tracking that rw_semaphore). If I implement vma_end_write() then it > will simply be something like: > > void vma_end_write(vma) > { > vma_assert_write_locked(vma); > vma->vm_lock_seq = UINT_MAX; > } > > so, vmlock_dep_map would not be involved. That's just weird; why would you not track vma_{start,end}_write() with the exclusive side of the 'rwsem' dep_map ? > If you want to track vma->vm_lock_seq with a separate lockdep, that > would be more complicated. Specifically for vma_end_write_all() that > would require us to call rwsem_release() on all locked vmas, however > we currently do not track individual locked vmas. vma_end_write_all() > allows us not to worry about tracking them, knowing that once we do > mmap_write_unlock() they all will get unlocked with one increment of > mm->mm_lock_seq. If your suggestion is to replace vma_end_write_all() > with vma_end_write() and unlock vmas individually across the mm code, > that would be a sizable effort. If that is indeed your ultimate goal, > I can do that as a separate project: introduce vma_end_write(), > gradually add them in required places (not yet sure how complex that > would be), then retire vma_end_write_all() and add a lockdep for > vma->vm_lock_seq. Yeah, so ultimately I think it would be clearer if you explicitly mark the point where the vma modification is 'done'. But I don't suppose we have to do that here.
