On Wed, Dec 18, 2024 at 8:18 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Wed, Dec 18, 2024 at 07:50:34AM -0800, Suren Baghdasaryan wrote: > > > I think vma_start_write() should be done inside > > vms_gather_munmap_vmas() for __mmap_prepare() to work correctly: > > > > __mmap_prepare > > vms_gather_munmap_vmas > > vms_clean_up_area // clears PTEs > > ... > > __mmap_complete > > vms_complete_munmap_vmas > > I'm unsure what exactly you mean; __split_vma() will start_write on the > one that is broken up and the rest won't actually change until > vms_complete_munmap_vmas(). Ah, sorry, I missed the write-locking in the __split_vma(). Looks like indeed vma_start_write() is not needed in vms_gather_munmap_vmas(). > > > If we do not write-lock the vmas inside vms_gather_munmap_vmas(), we > > will be clearing PTEs from under a discoverable vma. > > You will not. vms_complete_munmap_vmas() will call remove_vma() to > remove PTEs IIRC, and if you do start_write() and detach() before > dropping mmap_lock_write, you should be good. Ok, I think we will have to move mmap_write_downgrade() inside vms_complete_munmap_vmas() to be called after remove_vma(). vms_clear_ptes() is using vmas, so we can't move remove_vma() before mmap_write_downgrade(). > > > There might be other places like this too but I think we can move > > vma_mark_detach() like you suggested without moving vma_start_write() > > and that should be enough. > > I really don't see why you can't move vma_start_write() -- note that by > moving that after you've unhooked the vmas from the mm (which you have > by that point) you get the sync point you wanted. > >
