On Tue, Aug 25, 2020 at 02:54:28AM +0100, Al Viro wrote: > On Fri, Aug 21, 2020 at 09:20:54PM -0700, John Hubbard wrote: > > > Direct IO behavior: > > > > ITER_IOVEC: > > pin_user_pages_fast(); > > break; > > > > ITER_KVEC: // already elevated page refcount, leave alone > > ITER_BVEC: // already elevated page refcount, leave alone > > ITER_PIPE: // just, no :) > > Why? What's wrong with splice to O_DIRECT file?
Sorry - s/to/from/, obviously. To spell it out: consider generic_file_splice_read() behaviour when the source had been opened with O_DIRECT; you will get a call of ->read_iter() into ITER_PIPE destination. And it bloody well will hit iov_iter_get_pages() on common filesystems, to pick the pages we want to read into. So... what's wrong with having that "pin" primitive making sure the pages are there and referenced by the pipe?
