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 6/28/18 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)
>>              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

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?

Thanks,
Yang

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