On Mon, Sep 11, 2017 at 10:31:13AM +1000, Dave Chinner wrote: > splice does not go down the direct IO path, so iomap_dio_actor() > should never be handled a pipe as the destination for the IO data. > Indeed, splice read has to supply the pages to be put into the pipe, > which the DIO path does not do - it requires pages be supplied to > it. So I'm not sure why we'd care about pipe destination limitations > in the DIO path? splice doesn't give a rat's arse for direct IO; it's up to filesystem. generic_file_splice_read() simply sets up a pipe-backed iov_iter and calls ->read_iter(), period. iov_iter_get_pages() for pipe-backed destination does page allocation and inserts freshly allocated pages into pipe. copy_to_iter() does the same + copies data; copy_page_to_iter() grabs an extra reference to page and inserts it into pipe, not that O_DIRECT ->read_iter() had been likely to use the last one. Normally O_DIRECT would work just fine - pages get allocated, references to them put into pipe cyclic buffer *and* into a number of bio, bio would get submitted and once the IO is completed we unlock the pipe, making those pages available for readers. With minimal care it works just fine - all you really need is * cope with failing copy_to_... / iov_iter_get_pages(). Short read if we'd already gotten something, -EFAULT otherwise. That goes for pipe-backed same as for iovec-backed - any ->read_iter() that fails to handle that is already in trouble. * make sure that iov_iter_get_pages()/iov_iter_get_pages_alloc() is followed by iov_iter_advance() for the amount you've actually filled, before any subsequent copy_to_iter()/copy_page_to_iter() or return from ->read_iter(), whichever comes first. That includes the situation when you actually hadn't filled anything at all - just remember to do iov_iter_advance(to, 0) in that case. That's about the only extra requirement imposed by pipes and it's not hard to satisfy. Combination of iov_iter_advance() with iov_iter_revert() works as usual. Normally a filesystem doesn't need to care about splice at all - just use generic_file_splice_read() and be done with that. It will use the normal ->read_iter(), with whatever locking, etc., your filesystem would do on a normal read. -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
