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
