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

[Yang Shi]

On 6/28/18 12:10 PM, Yang Shi wrote:
> On 6/28/18 4:51 AM, Michal Hocko wrote:
> > 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)
> > > ÂÂÂÂ ÂÂÂ ÂÂÂ 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
> > ÂÂÂÂdown_write
> > ÂÂÂÂsplit & set up VM_DEAD
> > ÂÂÂÂdowngrade_write
> > ÂÂÂÂunmap
> > ÂÂÂÂupgrade_read
> > ÂÂÂÂzap ptes
> > ÂÂÂÂup_write

Promoting to write lock may be a trouble. There might be other users in 
the critical section with read lock, we have to wait them to finish.

> I'm supposed address space changing just can be done by mmap, mremap, 
> mprotect. If so, we may utilize the new VM_DEAD flag. If the VM_DEAD 
> flag is set for the vma, just return failure since it is being unmapped.
>
> Does it sounds reasonable?

It looks we just need care about MAP_FIXED (mmap) and MREMAP_FIXED 
(mremap), right?

How about letting them return -EBUSY or -EAGAIN to notify the 
application? This changes the behavior a little bit, MAP_FIXED and 
mremap may fail if they fail the race with munmap (if the mapping is 
larger than 1GB). I'm not sure if any multi-threaded application uses 
MAP_FIXED and MREMAP_FIXED very heavily which may run into the race 
condition. I guess it should be rare to meet all the conditions to 
trigger the race.

The programmer should be very cautious about MAP_FIXED.MREMAP_FIXED 
since they may corrupt its own address space as the man page noted.


Thanks,
Yang

> Thanks,
> Yang
>
> > looks terrible, no question about that, but we won't drop the mmap sem
> > at any time.