Bernard Metzler <BMT@xxxxxxxxxxxxxx> wrote: > > When transmitting data, call down into TCP using a single sendmsg with > > MSG_SPLICE_PAGES to indicate that content should be spliced rather than > > performing several sendmsg and sendpage calls to transmit header, data > > pages and trailer. > > > > To make this work, the data is assembled in a bio_vec array and attached to > > a BVEC-type iterator. The header and trailer (if present) are copied into > > page fragments that can be freed with put_page(). > > I like it a lot if it still keeps zero copy sendpage() semantics for > the cases the driver can make use of data transfers w/o copy. > Is 'msg.msg_flags |= MSG_SPLICE_PAGES' doing that magic? Yes. MSG_SPLICE_PAGES indicates that you want the socket to retain your buffer and pass it directly to the device. Note that it's just a hint, however, pages that are unspliceable (eg. they belong to the slab) will get copied into a page fragment instead. Further, if the device cannot support a vector, then the hint can be ignored and all the data can be copied as normal. > 'splicing' suggest just merging pages to me. 'splicing' as in what the splice system call does. Unfortunately, MSG_ZEROCOPY is already a (different) thing. > It would simplify the transmit code path substantially, also getting > rid of kmap_local_page()/kunmap_local() sequences for multi-fragment > sendmsg()'s. If the ITER_ITERLIST iterator is accepted, then siw would be able to do mix KVEC and BVEC iterators, e.g. what I did for sunrpc here: https://lore.kernel.org/linux-fsdevel/20230329141354.516864-42-dhowells@xxxxxxxxxx/T/#u This means that in siw_tx_hdt() where I made it copy data into page fragments using page_frag_memdup() and attach that to a bvec: hdr_len = c_tx->ctrl_len - c_tx->ctrl_sent; h = page_frag_memdup(NULL, hdr, hdr_len, GFP_NOFS, ULONG_MAX); if (!h) goto done; bvec_set_virt(&bvec[0], h, hdr_len); seg = 1; it can just set up a kvec instead. Unfortunately, it's not so easy to get rid of all of the kmap'ing as we need to do some of it to do the hashing. David
