Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping

[Michal Hocko]

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