Re: [PATCH RFC v2 00/19] fuse: fuse-over-io-uring

[Kent Overstreet <kent.overstreet@linux.dev>]

On Wed, Jun 12, 2024 at 06:15:57PM GMT, Bernd Schubert wrote:
> 
> 
> On 6/12/24 17:55, Kent Overstreet wrote:
> > On Wed, Jun 12, 2024 at 03:40:14PM GMT, Bernd Schubert wrote:
> > > On 6/12/24 16:19, Kent Overstreet wrote:
> > > > On Wed, Jun 12, 2024 at 03:53:42PM GMT, Bernd Schubert wrote:
> > > > > I will definitely look at it this week. Although I don't like the idea
> > > > > to have a new kthread. We already have an application thread and have
> > > > > the fuse server thread, why do we need another one?
> > > > 
> > > > Ok, I hadn't found the fuse server thread - that should be fine.
> > > > 
> > > > > > 
> > > > > > The next thing I was going to look at is how you guys are using splice,
> > > > > > we want to get away from that too.
> > > > > 
> > > > > Well, Ming Lei is working on that for ublk_drv and I guess that new approach
> > > > > could be adapted as well onto the current way of io-uring.
> > > > > It _probably_ wouldn't work with IORING_OP_READV/IORING_OP_WRITEV.
> > > > > 
> > > > > https://lore.gnuweeb.org/io-uring/20240511001214.173711-6-ming.lei@redhat.com/T/
> > > > > 
> > > > > > 
> > > > > > Brian was also saying the fuse virtio_fs code may be worth
> > > > > > investigating, maybe that could be adapted?
> > > > > 
> > > > > I need to check, but really, the majority of the new additions
> > > > > is just to set up things, shutdown and to have sanity checks.
> > > > > Request sending/completing to/from the ring is not that much new lines.
> > > > 
> > > > What I'm wondering is how read/write requests are handled. Are the data
> > > > payloads going in the same ringbuffer as the commands? That could work,
> > > > if the ringbuffer is appropriately sized, but alignment is a an issue.
> > > 
> > > That is exactly the big discussion Miklos and I have. Basically in my
> > > series another buffer is vmalloced, mmaped and then assigned to ring entries.
> > > Fuse meta headers and application payload goes into that buffer.
> > > In both kernel/userspace directions. io-uring only allows 80B, so only a
> > > really small request would fit into it.
> > 
> > Well, the generic ringbuffer would lift that restriction.
> 
> Yeah, kind of. Instead allocating the buffer in fuse, it would be now allocated
> in that code. At least all that setup code would be moved out of fuse. I will
> eventually come to your patches today.
> Now we only need to convince Miklos that your ring is better ;)
> 
> > 
> > > Legacy /dev/fuse has an alignment issue as payload follows directly as the fuse
> > > header - intrinsically fixed in the ring patches.
> > 
> > *nod*
> > 
> > That's the big question, put the data inline (with potential alignment
> > hassles) or manage (and map) a separate data structure.
> > 
> > Maybe padding could be inserted to solve alignment?
> 
> Right now I have this struct:
> 
> struct fuse_ring_req {
> 	union {
> 		/* The first 4K are command data */
> 		char ring_header[FUSE_RING_HEADER_BUF_SIZE];
> 
> 		struct {
> 			uint64_t flags;
> 
> 			/* enum fuse_ring_buf_cmd */
> 			uint32_t in_out_arg_len;
> 			uint32_t padding;
> 
> 			/* kernel fills in, reads out */
> 			union {
> 				struct fuse_in_header in;
> 				struct fuse_out_header out;
> 			};
> 		};
> 	};
> 
> 	char in_out_arg[];
> };
> 
> 
> Data go into in_out_arg, i.e. headers are padded by the union.
> I actually wonder if FUSE_RING_HEADER_BUF_SIZE should be page size
> and not a fixed 4K.

I would make the commands variable sized, so that commands with no data
buffers don't need padding, and then when you do have a data command you
only pad out that specific command so that the data buffer starts on a
page boundary.