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. > > 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. Ira
