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 > 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? > Honza >