On Wed 27-06-18 10:23:39, Yang Shi wrote: > > > On 6/27/18 12:24 AM, Michal Hocko wrote: > > On Tue 26-06-18 18:03:34, Yang Shi wrote: > > > > > > On 6/26/18 12:43 AM, Peter Zijlstra wrote: > > > > On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote: > > > > > By looking this deeper, we may not be able to cover all the unmapping range > > > > > for VM_DEAD, for example, if the start addr is in the middle of a vma. We > > > > > can't set VM_DEAD to that vma since that would trigger SIGSEGV for still > > > > > mapped area. > > > > > > > > > > splitting can't be done with read mmap_sem held, so maybe just set VM_DEAD > > > > > to non-overlapped vmas. Access to overlapped vmas (first and last) will > > > > > still have undefined behavior. > > > > Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. Acquire > > > > mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for > > > > writing, free everything left, drop mmap_sem. > > > > > > > > ? > > > > > > > > Sure, you acquire the lock 3 times, but both write instances should be > > > > 'short', and I suppose you can do a demote between 1 and 2 if you care. > > > Thanks, Peter. Yes, by looking the code and trying two different approaches, > > > it looks this approach is the most straight-forward one. > > Yes, you just have to be careful about the max vma count limit. > > Yes, we should just need copy what do_munmap does as below: > > if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count) > return -ENOMEM; > > If the mas map count limit has been reached, it will return failure before > zapping mappings. Yeah, but as soon as you drop the lock and retake it, somebody might have changed the adddress space and we might get inconsistency. So I am wondering whether we really need upgrade_read (to promote read to write lock) and do the down_write split & set up VM_DEAD downgrade_write unmap upgrade_read zap ptes up_write looks terrible, no question about that, but we won't drop the mmap sem at any time. -- Michal Hocko SUSE Labs
