On Wed 14-08-19 20:01:07, John Hubbard wrote: > On 8/14/19 5:02 PM, John Hubbard wrote: > > On 8/14/19 4:50 PM, Ira Weiny wrote: > > > On Tue, Aug 13, 2019 at 05:56:31PM -0700, John Hubbard wrote: > > > > On 8/13/19 5:51 PM, John Hubbard wrote: > > > > > On 8/13/19 2:08 PM, Ira Weiny wrote: > > > > > > On Mon, Aug 12, 2019 at 05:07:32PM -0700, John Hubbard wrote: > > > > > > > On 8/12/19 4:49 PM, Ira Weiny wrote: > > > > > > > > On Sun, Aug 11, 2019 at 06:50:44PM -0700, john.hubbard@xxxxxxxxx wrote: > > > > > > > > > From: John Hubbard <jhubbard@xxxxxxxxxx> > > > > > > > ... > > > > > > Finally, I struggle with converting everyone to a new call. It is more > > > > > > overhead to use vaddr_pin in the call above because now the GUP code is going > > > > > > to associate a file pin object with that file when in ODP we don't need that > > > > > > because the pages can move around. > > > > > > > > > > What if the pages in ODP are file-backed? > > > > > > > > > > > > > oops, strike that, you're right: in that case, even the file system case is covered. > > > > Don't mind me. :) > > > > > > Ok so are we agreed we will drop the patch to the ODP code? I'm going to keep > > > the FOLL_PIN flag and addition in the vaddr_pin_pages. > > > > > > > Yes. I hope I'm not overlooking anything, but it all seems to make sense to > > let ODP just rely on the MMU notifiers. > > > > 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. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
