On Fri, Aug 16, 2019 at 05:44:04PM +0200, Jan Kara wrote: > On Fri 16-08-19 10:47:21, Vlastimil Babka wrote: > > On 8/15/19 3:35 PM, Jan Kara wrote: > > >> > > >> 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. > > > > IMHO the munlock lesson told us about another one, that's in the end > > equivalent > > to 3) > > > > 4) pinning for struct page manipulation only => normal page references > > are fine > > Right, it's good to have this for clarity. > > > > 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... > > > > So after 1/2/3 is renamed/specially denoted, only 4) keeps the current > > interface? > > Well, munlock() code doesn't even use GUP, just follow_page(). I'd wait to > see what's left after handling cases 1), 2), and 3) to decide about the > interface for the remainder. >
For 3 we do not need to take a reference at all :) So just forget about 3 it does not exist. For 3 the reference is the reference the CPU page table has on the page and that's it. GUP is no longer involve in ODP or anything like that. Cheers, Jérôme
