Re: [WiP]: aio support for migrating pages (Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non_movable())

[Tang Chen <tangchen@cn.fujitsu.com>]

Hi Benjamin,

Are you still working on this problem ?

Thanks. :)

On 05/21/2013 10:27 AM, Benjamin LaHaise wrote:
> On Tue, May 21, 2013 at 10:07:52AM +0800, Tang Chen wrote:
> ....
>> I'm not saying using two callbacks before and after migration is better.
>> I don't want to use address_space_operations is because there is no such
>> member
>> for anonymous pages.
>
> That depends on the nature of the pinning.  For the general case of
> get_user_pages(), you're correct that it won't work for anonymous memory.
>
>> In your idea, using a file mapping will create a
>> address_space_operations. But
>> I really don't think we can modify the way of memory allocation for all the
>> subsystems who has this problem. Maybe not just aio and cma. That means if
>> you want to pin pages in memory, you have to use a file mapping. This makes
>> the memory allocation more complicated. And the idea should be known by all
>> the subsystem developers. Is that going to happen ?
>
> Different subsystems will need to use different approaches to fixing the
> issue.  I doubt any single approach will work for everything.
>
>> I also thought about reuse one field of struct page. But as you said, there
>> may not be many users of this functionality. Reusing a field of struct page
>> will make things more complicated and lead to high coupling.
>
> What happens when more than one subsystem tries to pin a particular page?
> What if it's a shared page rather than an anonymous page?
>
>> So, how about the other idea that Mel mentioned ?
>>
>> We create a 1-1 mapping of pinned page ranges and the pinner (subsystem
>> callbacks and data), maybe a global list or a hash table. And then, we can
>> find the callbacks.
>
> Maybe that is the simplest approach, but it's going to make get_user_pages()
> slower and more complicated (as if it wasn't already).  Maybe with all the
> bells and whistles of per-cpu data structures and such you can make it work,
> but I'm pretty sure someone running the large unmentionable benchmark will
> complain about the performance regressions you're going to introduce.  At
> least in the case of the AIO ring buffer, using the address_space approach
> doesn't introduce any new performance issues.  There's also the bigger
> question of if you can or cannot exclude get_user_pages_fast() from this.
> In short: you've got a lot more work on your hands to do.
>
>> Thanks. :)
>
> Cheers,
>
> 		-ben

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