Re: [PATCH v3 08/13] io_uring: implement fixed buffers registration similar to fixed files

[Pavel Begunkov <asml.silence@gmail.com>]

On 06/01/2021 19:46, Bijan Mottahedeh wrote:
> On 1/4/2021 6:43 PM, Pavel Begunkov wrote:
>> On 18/12/2020 18:07, Bijan Mottahedeh wrote:
>>> Apply fixed_rsrc functionality for fixed buffers support.
>>
>> git generated a pretty messy diff...
> 
> I had tried to break this up a few ways but it didn't work well because I think most of the code changes depend on the io_uring structure changes.  I can look again or if you some idea of how you want to split it, I can do that.
> 
>> Because it's do quiesce, fixed read/write access buffers from asynchronous
>> contexts without synchronisation. That won't work anymore, so
>>
>> 1. either we save it in advance, that would require extra req_async
>> allocation for linked fixed rw
>>
>> 2. or synchronise whenever async. But that would mean that a request
>> may get and do IO on two different buffers, that's rotten.
>>
>> 3. do mixed -- lazy, but if do IO then alloc.
>>
>> 3.5 also "synchronise" there would mean uring_lock, that's not welcome,
>> but we can probably do rcu.
> 
> Are you referring to a case where a fixed buffer request can be submitted from async context while those buffers are being unregistered, or something like that?
> 
>> Let me think of a patch...

The most convenient API would be [1], it selects a buffer during
submission, but allocates if needs to go async or for all linked
requests.

[2] should be correct from the kernel perspective (no races), it
also solves doing IO on 2 different buffers, that's nasty (BTW,
[1] solves this problem naturally). However, a buffer might be
selected async, but the following can happen, and user should
wait for request completion before removing a buffer.

1. register buf id=0
2. syscall io_uring_enter(submit=RW_FIXED,buf_id=0,IOSQE_ASYNC)
3. unregister buffers
4. the request may not find the buffer and fail.

Not very convenient + can actually add overhead on the userspace
side, can be even some heavy synchronisation.

uring_lock in [2] is not nice, but I think I can replace it
with rcu, probably can even help with sharing, but I need to
try to implement to be sure.

So that's an open question what API to have.
Neither of diffs is tested.

[1]
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7e35283fc0b1..2171836a9ce3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -826,6 +826,7 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_file		= 1,
 		.unbound_nonreg_file	= 1,
 		.pollin			= 1,
+		.needs_async_data	= 1,
 		.plug			= 1,
 		.async_size		= sizeof(struct io_async_rw),
 		.work_flags		= IO_WQ_WORK_BLKCG | IO_WQ_WORK_MM,
@@ -835,6 +836,7 @@ static const struct io_op_def io_op_defs[] = {
 		.hash_reg_file		= 1,
 		.unbound_nonreg_file	= 1,
 		.pollout		= 1,
+		.needs_async_data	= 1,
 		.plug			= 1,
 		.async_size		= sizeof(struct io_async_rw),
 		.work_flags		= IO_WQ_WORK_BLKCG | IO_WQ_WORK_FSIZE |



[2]
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7e35283fc0b1..31560b879fb3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3148,7 +3148,12 @@ static ssize_t io_import_iovec(int rw, struct io_kiocb *req,
 	opcode = req->opcode;
 	if (opcode == IORING_OP_READ_FIXED || opcode == IORING_OP_WRITE_FIXED) {
 		*iovec = NULL;
-		return io_import_fixed(req, rw, iter);
+
+		io_ring_submit_lock(req->ctx, needs_lock);
+		lockdep_assert_held(&req->ctx->uring_lock);
+		ret = io_import_fixed(req, rw, iter);
+		io_ring_submit_unlock(req->ctx, needs_lock);
+		return ret;
 	}
 
 	/* buffer index only valid with fixed read/write, or buffer select  */
@@ -3638,7 +3643,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 copy_iov:
 		/* some cases will consume bytes even on error returns */
 		iov_iter_revert(iter, io_size - iov_iter_count(iter));
-		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
+		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
 		if (!ret)
 			return -EAGAIN;
 	}


-- 
Pavel Begunkov