Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping

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

On 6/29/18 8:15 PM, Andrew Morton wrote:
> On Fri, 29 Jun 2018 19:28:15 -0700 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>>
>>> we're adding a bunch of code to 32-bit kernels which will never be
>>> executed.
>>>
>>> I'm thinking it would be better to be much more explicit with "#ifdef
>>> CONFIG_64BIT" in this code, rather than relying upon the above magic.
>>>
>>> But I tend to think that the fact that we haven't solved anything on
>>> locked vmas or on uprobed mappings is a shostopper for the whole
>>> approach :(
>> I agree it is not that perfect. But, it still could improve the most use
>> cases.
> Well, those unaddressed usecases will need to be fixed at some point.

Yes, definitely.

> What's our plan for that?

As I mentioned in the earlier email, locked and hugetlb cases might be 
able to be solved by separating vm_flags update and actual unmap. I will 
look into it further later.

 From my point of view, uprobe mapping sounds not that vital.

>
> Would one of your earlier designs have addressed all usecases?  I
> expect the dumb unmap-a-little-bit-at-a-time approach would have?

Yes. The v1 design does unmap with holding write map_sem. So, the 
vm_flags update is not a problem.

Thanks,
Yang

>
>> For the locked vmas and hugetlb vmas, unmapping operations need modify
>> vm_flags. But, I'm wondering we might be able to separate unmap and
>> vm_flags update. Because we know they will be unmapped right away, the
>> vm_flags might be able to be updated in write mmap_sem critical section
>> before the actual unmap is called or after it. This is just off the top
>> of my head.
>>
>> For uprobed mappings, I'm not sure how vital it is to this case.
>>
>> Thanks,
>> Yang
>>