On Wed, Sep 07, 2016 at 09:59:17AM +1000, Dave Chinner wrote: > Not sure I quite follow - where do the pages come from? Do we > allocate new pages that get put into the bvec, then run the read > which copies data from the page cache page into them, then hand > those pages in the bvec to the pipe? Nope. generic_file_read_iter() (do_generic_file_read(), in the end) finds them in page cache, or allocates and sticks them into pagecache, makes sure that they are uptodate, etc. And passes them to copy_page_to_iter(), which would, for this iov_iter flavour, just grab a reference to page and stash it into bvec. There's your zero-copy, exactly as it works now. Only __generic_file_splice_read() open-codes everything ->read_iter() would do, sans the locks filesystem would need. > This has the same "data copy in the splice read path" as the above > interface. However, I suspect that this interface could actually be > used for zero copy (by stealing pages from the page cache rather > than allocating new pages and copying), so it may be a better way to > proceed... For copy_page_to_iter() we have a page; for copy_to_iter() the data comes from hell knows what - kmalloc'ed array into which we'd decrypted something, results of sprintf() into on-stack array, etc. So the counterparts of copy_to_iter() callers must be non-zerocopy. copy_page_to_iter() is the potential zerocopy path and we do get zerocopy there that way. -- 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
