On 3/17/23 2:14?AM, Ming Lei wrote: > On Tue, Mar 14, 2023 at 08:57:11PM +0800, Ming Lei wrote: >> Hello, >> >> Add IORING_OP_FUSED_CMD, it is one special URING_CMD, which has to >> be SQE128. The 1st SQE(master) is one 64byte URING_CMD, and the 2nd >> 64byte SQE(slave) is another normal 64byte OP. For any OP which needs >> to support slave OP, io_issue_defs[op].fused_slave needs to be set as 1, >> and its ->issue() can retrieve/import buffer from master request's >> fused_cmd_kbuf. The slave OP is actually submitted from kernel, part of >> this idea is from Xiaoguang's ublk ebpf patchset, but this patchset >> submits slave OP just like normal OP issued from userspace, that said, >> SQE order is kept, and batching handling is done too. >> >> Please see detailed design in commit log of the 2th patch, and one big >> point is how to handle buffer ownership. >> >> With this way, it is easy to support zero copy for ublk/fuse device. >> >> Basically userspace can specify any sub-buffer of the ublk block request >> buffer from the fused command just by setting 'offset/len' >> in the slave SQE for running slave OP. This way is flexible to implement >> io mapping: mirror, stripped, ... >> >> The 3th & 4th patches enable fused slave support for the following OPs: >> >> OP_READ/OP_WRITE >> OP_SEND/OP_RECV/OP_SEND_ZC >> >> The other ublk patches cleans ublk driver and implement fused command >> for supporting zero copy. >> >> Follows userspace code: >> >> https://github.com/ming1/ubdsrv/tree/fused-cmd-zc-v2 >> >> All three(loop, nbd and qcow2) ublk targets have supported zero copy by passing: >> >> ublk add -t [loop|nbd|qcow2] -z .... >> >> Basic fs mount/kernel building and builtin test are done, and also not >> observe regression on xfstest test over ublk-loop with zero copy. >> >> Also add liburing test case for covering fused command based on miniublk >> of blktest: >> >> https://github.com/ming1/liburing/commits/fused_cmd_miniublk >> >> Performance improvement is obvious on memory bandwidth >> related workloads, such as, 1~2X improvement on 64K/512K BS >> IO test on loop with ramfs backing file. >> >> Any comments are welcome! >> >> V3: >> - fix build warning reported by kernel test robot >> - drop patch for checking fused flags on existed drivers with >> ->uring_command(), which isn't necessary, since we do not do that >> when adding new ioctl or uring command >> - inline io_init_rq() for core code, so just export io_init_slave_req >> - return result of failed slave request unconditionally since REQ_F_CQE_SKIP >> will be cleared >> - pass xfstest over ublk-loop > > Hello Jens and Guys, > > I have been working on io_uring zero copy support for ublk/fuse for a while, and > I appreciate you may share any thoughts on this patchset or approach? I'm a bit split on this one, as I really like (and want) the feature. ublk has become popular pretty quickly, and it makes a LOT of sense to support zero copy for it. At the same time, I'm not really a huge fan of the fused commands... They seem too specialized to be useful for other things, and it'd be a shame to do something like that only for it later to be replaced by a generic solution. And then we're stuck with supporting fused commands forever, not sure I like that prospect. Both Pavel and Xiaoguang voiced similar concerns, and I think it may be worth spending a bit more time on figuring out if splice can help us here. David Howells currently has a lot going on in that area too. So while I'd love to see this feature get queued up right now, I also don't want to prematurely do so. Can we split out the fixes from this series into a separate series that we can queue up now? That would also help shrink the patchset, which is always a win for review. -- Jens Axboe
