On 25.02.22 09:50, John Hubbard wrote: > Hi, > > Summary: > > This puts some prerequisites in place, including a CONFIG parameter, > making it possible to start converting and testing the Direct IO part of > each filesystem, from get_user_pages_fast(), to pin_user_pages_fast(). > > It will take "a few" kernel releases to get the whole thing done. > > Details: > > As part of fixing the "get_user_pages() + file-backed memory" problem > [1], and to support various COW-related fixes as well [2], we need to > convert the Direct IO code from get_user_pages_fast(), to > pin_user_pages_fast(). Because pin_user_pages*() calls require a > corresponding call to unpin_user_page(), the conversion is more > elaborate than just substitution. > > Further complicating the conversion, the block/bio layers get their > Direct IO pages via iov_iter_get_pages() and iov_iter_get_pages_alloc(), > each of which has a large number of callers. All of those callers need > to be audited and changed so that they call unpin_user_page(), rather > than put_page(). vmsplice is another candidate that uses iov_iter_get_pages() and should be converted to FOLL_PIN. For that particular user, we have to also pass FOLL_LONGTERM -- vmsplice as it stands can block memory hotunplug / CMA / ... for all eternity. -- Thanks, David / dhildenb
