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

[Yang Shi <yang.shi@linux.alibaba.com>]

On 6/29/18 4:39 AM, Michal Hocko wrote:
> On Thu 28-06-18 17:59:25, Yang Shi wrote:
>>
>> 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)
>>>>>  A A A A  A A A  A A A  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
>>>>  A A A A down_write
>>>>  A A A A split & set up VM_DEAD
>>>>  A A A A downgrade_write
>>>>  A A A A unmap
>>>>  A A A A upgrade_read
>>>>  A A A A zap ptes
>>>>  A A A A up_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.
> Yes. Is that a problem though?

Not a problem, but just not sure how complicated it would be. 
Considering all the lock debug/lockdep stuff.

And, the behavior smells like rcu.

>   
>>> 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?
> Well, non of those is documented to return EBUSY and EAGAIN already has
> a meaning for locked memory.
>
>> 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.
> Well, I suspect you are overcomplicating this a bit. This should be
> really straightforward thing - well except for VM_DEAD which is quite
> tricky already. We should rather not spread this trickyness outside of
> the #PF path. And I would even try hard to start that part simple to see
> whether it actually matters. Relying on races between threads without
> any locking is quite questionable already. Nobody has pointed to a sane
> usecase so far.

I agree to keep it as simple as possible then see if it matters or not. 
So, in v3 I will just touch the page fault path.