Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()

[John Hubbard <jhubbard@nvidia.com>]

On 8/15/19 10:32 AM, Ira Weiny wrote:
> On Thu, Aug 15, 2019 at 03:35:10PM +0200, Jan Kara wrote:
>> On Thu 15-08-19 15:26:22, Jan Kara wrote:
>>> On Wed 14-08-19 20:01:07, John Hubbard wrote:
>>>> On 8/14/19 5:02 PM, John Hubbard wrote:
>>>>
>>>> Hold on, I *was* forgetting something: this was a two part thing, and
>>>> you're conflating the two points, but they need to remain separate and
>>>> distinct. There were:
>>>>
>>>> 1. FOLL_PIN is necessary because the caller is clearly in the use case that
>>>> requires it--however briefly they might be there. As Jan described it,
>>>>
>>>> "Anything that gets page reference and then touches page data (e.g.
>>>> direct IO) needs the new kind of tracking so that filesystem knows
>>>> someone is messing with the page data." [1]
>>>
>>> So when the GUP user uses MMU notifiers to stop writing to pages whenever
>>> they are writeprotected with page_mkclean(), they don't really need page
>>> pin - their access is then fully equivalent to any other mmap userspace
>>> access and filesystem knows how to deal with those. I forgot out this case
>>> when I wrote the above sentence.
>>>
>>> So to sum up there are three cases:
>>> 1) DIO case - GUP references to pages serving as DIO buffers are needed for
>>>    relatively short time, no special synchronization with page_mkclean() or
>>>    munmap() => needs FOLL_PIN
>>> 2) RDMA case - GUP references to pages serving as DMA buffers needed for a
>>>    long time, no special synchronization with page_mkclean() or munmap()
>>>    => needs FOLL_PIN | FOLL_LONGTERM
>>>    This case has also a special case when the pages are actually DAX. Then
>>>    the caller additionally needs file lease and additional file_pin
>>>    structure is used for tracking this usage.
>>> 3) ODP case - GUP references to pages serving as DMA buffers, MMU notifiers
>>>    used to synchronize with page_mkclean() and munmap() => normal page
>>>    references are fine.
>>

Thanks Jan, once again, for clarifying all of this!

>> I want to add that I'd like to convert users in cases 1) and 2) from using
>> GUP to using differently named function. Users in case 3) can stay as they
>> are for now although ultimately I'd like to denote such use cases in a
>> special way as well...
>>
> 
> Ok just to make this clear I threw up my current tree with your patches here:
> 
> https://github.com/weiny2/linux-kernel/commits/mmotm-rdmafsdax-b0-v4
> 
> I'm talking about dropping the final patch:
> 05fd2d3afa6b rdma/umem_odp: Use vaddr_pin_pages_remote() in ODP
> 
> The other 2 can stay.  I split out the *_remote() call.  We don't have a user
> but I'll keep it around for a bit.
> 
> This tree is still WIP as I work through all the comments.  So I've not changed
> names or variable types etc...  Just wanted to settle this.
> 

Right. And now that ODP is not a user, I'll take a quick look through my other
call site conversions and see if I can find an easy one, to include here as
the first user of vaddr_pin_pages_remote(). I'll send it your way if that
works out.


thanks,
-- 
John Hubbard
NVIDIA