On 10/29/24 9:08 PM, Ming Lei wrote: > On Tue, Oct 29, 2024 at 08:43:39PM -0600, Jens Axboe wrote: >> On 10/29/24 8:03 PM, Ming Lei wrote: >>> On Tue, Oct 29, 2024 at 03:26:37PM -0600, Jens Axboe wrote: >>>> On 10/29/24 2:06 PM, Jens Axboe wrote: >>>>> On 10/29/24 1:18 PM, Jens Axboe wrote: >>>>>> Now, this implementation requires a user buffer, and as far as I'm told, >>>>>> you currently have kernel buffers on the ublk side. There's absolutely >>>>>> no reason why kernel buffers cannot work, we'd most likely just need to >>>>>> add a IORING_RSRC_KBUFFER type to handle that. My question here is how >>>>>> hard is this requirement? Reason I ask is that it's much simpler to work >>>>>> with userspace buffers. Yes the current implementation maps them >>>>>> everytime, we could certainly change that, however I don't see this >>>>>> being an issue. It's really no different than O_DIRECT, and you only >>>>>> need to map them once for a read + whatever number of writes you'd need >>>>>> to do. If a 'tag' is provided for LOCAL_BUF, it'll post a CQE whenever >>>>>> that buffer is unmapped. This is a notification for the application that >>>>>> it's done using the buffer. For a pure kernel buffer, we'd either need >>>>>> to be able to reference it (so that we KNOW it's not going away) and/or >>>>>> have a callback associated with the buffer. >>>>> >>>>> Just to expand on this - if a kernel buffer is absolutely required, for >>>>> example if you're inheriting pages from the page cache or other >>>>> locations you cannot control, we would need to add something ala the >>>>> below: >>>> >>>> Here's a more complete one, but utterly untested. But it does the same >>>> thing, mapping a struct request, but it maps it to an io_rsrc_node which >>>> in turn has an io_mapped_ubuf in it. Both BUFFER and KBUFFER use the >>>> same type, only the destruction is different. Then the callback provided >>>> needs to do something ala: >>>> >>>> struct io_mapped_ubuf *imu = node->buf; >>>> >>>> if (imu && refcount_dec_and_test(&imu->refs)) >>>> kvfree(imu); >>>> >>>> when it's done with the imu. Probably an rsrc helper should just be done >>>> for that, but those are details. >>>> >>>> diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c >>>> index 9621ba533b35..050868a4c9f1 100644 >>>> --- a/io_uring/rsrc.c >>>> +++ b/io_uring/rsrc.c >>>> @@ -8,6 +8,8 @@ >>>> #include <linux/nospec.h> >>>> #include <linux/hugetlb.h> >>>> #include <linux/compat.h> >>>> +#include <linux/bvec.h> >>>> +#include <linux/blk-mq.h> >>>> #include <linux/io_uring.h> >>>> >>>> #include <uapi/linux/io_uring.h> >>>> @@ -474,6 +476,9 @@ void io_free_rsrc_node(struct io_rsrc_node *node) >>>> if (node->buf) >>>> io_buffer_unmap(node->ctx, node); >>>> break; >>>> + case IORING_RSRC_KBUFFER: >>>> + node->kbuf_fn(node); >>>> + break; >>> >>> Here 'node' is freed later, and it may not work because ->imu is bound >>> with node. >> >> Not sure why this matters? imu can be bound to any node (and has a >> separate ref), but the node will remain for as long as the submission >> runs. It has to, because the last reference is put when submission of >> all requests in that series ends. > > Fine, how is the imu found from OP? Not see related code to add the > allocated node into submission_state or ctx->buf_table. Just didn't do that, see the POC test patch I did for rw for just grabbing the fixed one in io_submit_state. Really depends on how many we'd need - if it's just 1 per submit, then whatever I had would work and the OP just needs to know to look there. > io_rsrc_node_lookup() needs to find the buffer any way, right? That's for table lookup, for the POC there's just the one node hence nothing really to lookup. It's either rsrc_empty_node, or a valid node. >>> I think the reference should be in `node` which need to be live if any >>> consumer OP isn't completed. >> >> That is how it works... io_req_assign_rsrc_node() will assign a node to >> a request, which will be there until the request completes. >> >>>> + node->buf = imu; >>>> + node->kbuf_fn = kbuf_fn; >>>> + return node; >>> >>> Also this function needs to register the buffer to table with one >>> pre-defined buf index, then the following request can use it by >>> the way of io_prep_rw_fixed(). >> >> It should not register it with the table, the whole point is to keep >> this node only per-submission discoverable. If you're grabbing random >> request pages, then it very much is a bit finicky and needs to be of >> limited scope. > > There can be more than 1 buffer uses in single submission, can you share > how OP finds the specific buffer with ->buf_index from submission state? > This part is missed in your patch. If we need more than one, then yeah we'd need an index rather than just a single pointer. Doesn't really change the mechanics, you'd need to provide an index like with ->buf_index. It's not missed in the patch, it's really just a POC patch to show how it can be done, by no means a done solution! But we can certainly get it there. >> Each request type would need to support it. For normal read/write, I'd >> suggest just adding IORING_OP_READ_LOCAL and WRITE_LOCAL to do that. >> >>> If OP dependency can be avoided, I think this approach is fine, >>> otherwise I still suggest sqe group. Not only performance, but >>> application becomes too complicated. >> >> You could avoid the OP dependency with just a flag, if you really wanted >> to. But I'm not sure it makes a lot of sense. And it's a hell of a lot > > Yes, IO_LINK won't work for submitting multiple IOs concurrently, extra > syscall makes application too complicated, and IO latency is increased. It's really not a big deal to prepare-and-submit the dependencies separately, but at the same time, I don't think it'd be a bad idea to support eg 2 local buffers per submit. Or whatever we need there. This is more from a usability point of view, because the rest of the machinery is so much more expensive than a single extra syscall that the latter is not goinbg to affect IO latencies at all. -- Jens Axboe
