[RFC 0/4] open/accept directly into io_uring fixed file table

[RFC 0/4] open/accept directly into io_uring fixed file table

[Pavel Begunkov]

Implement an old idea allowing open/accept io_uring requests to register
a newly created file as a io_uring's fixed file instead of placing it
into a task's file table. The switching is encoded in io_uring's SQEs
by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
think we need more, but may be a good idea to scrap u32 somewhere
instead.

From the net side only needs a function doing __sys_accept4_file()
but not installing fd, see 2/4.

Only RFC for now, the new functionality is tested only for open yet.
I hope we can remember the author of the idea to add attribution.

Pavel Begunkov (4):
  io_uring: allow open directly into fixed fd table
  net: add an accept helper not installing fd
  io_uring: hand code io_accept()' fd installing
  io_uring: accept directly into fixed file table

 fs/io_uring.c                 | 113 +++++++++++++++++++++++++++++-----
 include/linux/socket.h        |   3 +
 include/uapi/linux/io_uring.h |   2 +
 net/socket.c                  |  71 +++++++++++----------
 4 files changed, 138 insertions(+), 51 deletions(-)

-- 
2.32.0

[PATCH 1/4] io_uring: allow open directly into fixed fd table

[Pavel Begunkov]

Instead of opening a file into a process's file table as usual and then
registering the fd within io_uring, we can skip the first step and place
it directly into io_uring's fixed file table. Add support for that for
both OPENAT opcodes.

The behaviour is controlled by setting sqe->file_index, where 0 implies
the old behaviour, and non-0 values is translated into index
(file_index-1). The field is u16, hence sets a limit on how many slots
it covers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c                 | 75 ++++++++++++++++++++++++++++++-----
 include/uapi/linux/io_uring.h |  2 +
 2 files changed, 68 insertions(+), 9 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 599b11152505..17d2c0eb89dc 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1075,6 +1075,8 @@ static bool io_poll_remove_waitqs(struct io_kiocb *req);
 static int io_req_prep_async(struct io_kiocb *req);
 
 static void io_fallback_req_func(struct work_struct *unused);
+static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
+				 unsigned int issue_flags);
 
 static struct kmem_cache *req_cachep;
 
@@ -3745,11 +3747,10 @@ static int __io_openat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
 	const char __user *fname;
 	int ret;
 
-	if (unlikely(sqe->ioprio || sqe->buf_index))
+	if (unlikely(sqe->ioprio))
 		return -EINVAL;
 	if (unlikely(req->flags & REQ_F_FIXED_FILE))
 		return -EBADF;
-
 	/* open.how should be already initialised */
 	if (!(req->open.how.flags & O_PATH) && force_o_largefile())
 		req->open.how.flags |= O_LARGEFILE;
@@ -3762,6 +3763,10 @@ static int __io_openat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
 		req->open.filename = NULL;
 		return ret;
 	}
+	req->buf_index = READ_ONCE(sqe->file_index);
+	if (req->buf_index && (req->open.how.flags & O_CLOEXEC))
+		return -EINVAL;
+
 	req->open.nofile = rlimit(RLIMIT_NOFILE);
 	req->flags |= REQ_F_NEED_CLEANUP;
 	return 0;
@@ -3804,8 +3809,8 @@ static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct open_flags op;
 	struct file *file;
-	bool nonblock_set;
-	bool resolve_nonblock;
+	bool resolve_nonblock, nonblock_set;
+	bool fixed = !!req->buf_index;
 	int ret;
 
 	ret = build_open_flags(&req->open.how, &op);
@@ -3824,9 +3829,11 @@ static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
 		op.open_flag |= O_NONBLOCK;
 	}
 
-	ret = __get_unused_fd_flags(req->open.how.flags, req->open.nofile);
-	if (ret < 0)
-		goto err;
+	if (!fixed) {
+		ret = __get_unused_fd_flags(req->open.how.flags, req->open.nofile);
+		if (ret < 0)
+			goto err;
+	}
 
 	file = do_filp_open(req->open.dfd, req->open.filename, &op);
 	if (IS_ERR(file)) {
@@ -3835,7 +3842,8 @@ static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
 		 * marginal gain for something that is now known to be a slower
 		 * path. So just put it, and we'll get a new one when we retry.
 		 */
-		put_unused_fd(ret);
+		if (!fixed)
+			put_unused_fd(ret);
 
 		ret = PTR_ERR(file);
 		/* only retry if RESOLVE_CACHED wasn't already set by application */
@@ -3848,7 +3856,11 @@ static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
 	if ((issue_flags & IO_URING_F_NONBLOCK) && !nonblock_set)
 		file->f_flags &= ~O_NONBLOCK;
 	fsnotify_open(file);
-	fd_install(ret, file);
+
+	if (!fixed)
+		fd_install(ret, file);
+	else
+		ret = io_install_fixed_file(req, file, issue_flags);
 err:
 	putname(req->open.filename);
 	req->flags &= ~REQ_F_NEED_CLEANUP;
@@ -7776,6 +7788,50 @@ static int io_sqe_file_register(struct io_ring_ctx *ctx, struct file *file,
 #endif
 }
 
+static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
+				 unsigned int issue_flags)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+	int i = req->buf_index - 1;
+	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
+	struct io_fixed_file *file_slot;
+	int ret = -EBADF;
+
+	if (WARN_ON_ONCE(req->buf_index == 0))
+		goto err;
+
+	io_ring_submit_lock(ctx, !force_nonblock);
+	if (file->f_op == &io_uring_fops)
+		goto err;
+	ret = -ENXIO;
+	if (!ctx->file_data)
+		goto err;
+	ret = -EINVAL;
+	if (i > ctx->nr_user_files)
+		goto err;
+
+	i = array_index_nospec(i, ctx->nr_user_files);
+	file_slot = io_fixed_file_slot(&ctx->file_table, i);
+	ret = -EEXIST;
+	if (file_slot->file_ptr)
+		goto err;
+
+	*io_get_tag_slot(ctx->file_data, i) = 0;
+	io_fixed_file_set(file_slot, file);
+	ret = io_sqe_file_register(ctx, file, i);
+	if (ret) {
+		file_slot->file_ptr = 0;
+		goto err;
+	}
+
+	ret = 0;
+err:
+	io_ring_submit_unlock(ctx, !force_nonblock);
+	if (ret)
+		fput(file);
+	return ret;
+}
+
 static int io_queue_rsrc_removal(struct io_rsrc_data *data, unsigned idx,
 				 struct io_rsrc_node *node, void *rsrc)
 {
@@ -10231,6 +10287,7 @@ static int __init io_uring_init(void)
 	BUILD_BUG_SQE_ELEM(32, __u64,  user_data);
 	BUILD_BUG_SQE_ELEM(40, __u16,  buf_index);
 	BUILD_BUG_SQE_ELEM(40, __u16,  buf_group);
+	BUILD_BUG_SQE_ELEM(40, __u16,  file_index);
 	BUILD_BUG_SQE_ELEM(42, __u16,  personality);
 	BUILD_BUG_SQE_ELEM(44, __s32,  splice_fd_in);
 
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 79126d5cd289..f105deb4da1d 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -52,6 +52,8 @@ struct io_uring_sqe {
 		__u16	buf_index;
 		/* for grouped buffer selection */
 		__u16	buf_group;
+		/* index into fixed files */
+		__u16	file_index;
 	} __attribute__((packed));
 	/* personality to use, if used */
 	__u16	personality;
-- 
2.32.0

[PATCH 2/4] net: add an accept helper not installing fd

[Pavel Begunkov]

Introduce and reuse a helper that acts similarly to __sys_accept4_file()
but returns struct file instead of installing file descriptor. Will be
used by io_uring.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 include/linux/socket.h |  3 ++
 net/socket.c           | 71 ++++++++++++++++++++++--------------------
 2 files changed, 40 insertions(+), 34 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 0d8e3dcb7f88..d3c1a42a2edd 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -421,6 +421,9 @@ extern int __sys_accept4_file(struct file *file, unsigned file_flags,
 			struct sockaddr __user *upeer_sockaddr,
 			 int __user *upeer_addrlen, int flags,
 			 unsigned long nofile);
+extern struct file *do_accept(struct file *file, unsigned file_flags,
+			      struct sockaddr __user *upeer_sockaddr,
+			      int __user *upeer_addrlen, int flags);
 extern int __sys_accept4(int fd, struct sockaddr __user *upeer_sockaddr,
 			 int __user *upeer_addrlen, int flags);
 extern int __sys_socket(int family, int type, int protocol);
diff --git a/net/socket.c b/net/socket.c
index bd9233da2497..2857206453f9 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1715,32 +1715,22 @@ SYSCALL_DEFINE2(listen, int, fd, int, backlog)
 	return __sys_listen(fd, backlog);
 }
 
-int __sys_accept4_file(struct file *file, unsigned file_flags,
+struct file *do_accept(struct file *file, unsigned file_flags,
 		       struct sockaddr __user *upeer_sockaddr,
-		       int __user *upeer_addrlen, int flags,
-		       unsigned long nofile)
+		       int __user *upeer_addrlen, int flags)
 {
 	struct socket *sock, *newsock;
 	struct file *newfile;
-	int err, len, newfd;
+	int err, len;
 	struct sockaddr_storage address;
 
-	if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
-		return -EINVAL;
-
-	if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
-		flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;
-
 	sock = sock_from_file(file);
-	if (!sock) {
-		err = -ENOTSOCK;
-		goto out;
-	}
+	if (!sock)
+		return ERR_PTR(-ENOTSOCK);
 
-	err = -ENFILE;
 	newsock = sock_alloc();
 	if (!newsock)
-		goto out;
+		return ERR_PTR(-ENFILE);
 
 	newsock->type = sock->type;
 	newsock->ops = sock->ops;
@@ -1751,18 +1741,9 @@ int __sys_accept4_file(struct file *file, unsigned file_flags,
 	 */
 	__module_get(newsock->ops->owner);
 
-	newfd = __get_unused_fd_flags(flags, nofile);
-	if (unlikely(newfd < 0)) {
-		err = newfd;
-		sock_release(newsock);
-		goto out;
-	}
 	newfile = sock_alloc_file(newsock, flags, sock->sk->sk_prot_creator->name);
-	if (IS_ERR(newfile)) {
-		err = PTR_ERR(newfile);
-		put_unused_fd(newfd);
-		goto out;
-	}
+	if (IS_ERR(newfile))
+		return newfile;
 
 	err = security_socket_accept(sock, newsock);
 	if (err)
@@ -1787,16 +1768,38 @@ int __sys_accept4_file(struct file *file, unsigned file_flags,
 	}
 
 	/* File flags are not inherited via accept() unlike another OSes. */
-
-	fd_install(newfd, newfile);
-	err = newfd;
-out:
-	return err;
+	return newfile;
 out_fd:
 	fput(newfile);
-	put_unused_fd(newfd);
-	goto out;
+	return ERR_PTR(err);
+}
+
+int __sys_accept4_file(struct file *file, unsigned file_flags,
+		       struct sockaddr __user *upeer_sockaddr,
+		       int __user *upeer_addrlen, int flags,
+		       unsigned long nofile)
+{
+	struct file *newfile;
+	int newfd;
 
+	if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
+		return -EINVAL;
+
+	if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
+		flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;
+
+	newfd = __get_unused_fd_flags(flags, nofile);
+	if (unlikely(newfd < 0))
+		return newfd;
+
+	newfile = do_accept(file, file_flags, upeer_sockaddr, upeer_addrlen,
+			    flags);
+	if (IS_ERR(newfile)) {
+		put_unused_fd(newfd);
+		return PTR_ERR(newfile);
+	}
+	fd_install(newfd, newfile);
+	return newfd;
 }
 
 /*
-- 
2.32.0

[PATCH 3/4] io_uring: hand code io_accept()' fd installing

[Pavel Begunkov]

Make io_accept() to handle file descriptor allocations and installation.
A preparation patch for bypassing file tables.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 17d2c0eb89dc..fd0dcee251b0 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4682,6 +4682,11 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	accept->addr_len = u64_to_user_ptr(READ_ONCE(sqe->addr2));
 	accept->flags = READ_ONCE(sqe->accept_flags);
 	accept->nofile = rlimit(RLIMIT_NOFILE);
+
+	if (accept->flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
+		return -EINVAL;
+	if (SOCK_NONBLOCK != O_NONBLOCK && (accept->flags & SOCK_NONBLOCK))
+		accept->flags = (accept->flags & ~SOCK_NONBLOCK) | O_NONBLOCK;
 	return 0;
 }
 
@@ -4690,20 +4695,28 @@ static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
 	struct io_accept *accept = &req->accept;
 	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
 	unsigned int file_flags = force_nonblock ? O_NONBLOCK : 0;
-	int ret;
+	struct file *file;
+	int ret, fd;
 
 	if (req->file->f_flags & O_NONBLOCK)
 		req->flags |= REQ_F_NOWAIT;
 
-	ret = __sys_accept4_file(req->file, file_flags, accept->addr,
-					accept->addr_len, accept->flags,
-					accept->nofile);
-	if (ret == -EAGAIN && force_nonblock)
-		return -EAGAIN;
-	if (ret < 0) {
+	fd = __get_unused_fd_flags(accept->flags, accept->nofile);
+	if (unlikely(fd < 0))
+		return fd;
+
+	file = do_accept(req->file, file_flags, accept->addr, accept->addr_len,
+			 accept->flags);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		if (ret == -EAGAIN && force_nonblock)
+			return -EAGAIN;
 		if (ret == -ERESTARTSYS)
 			ret = -EINTR;
 		req_set_fail(req);
+	} else {
+		fd_install(fd, file);
+		ret = fd;
 	}
 	__io_req_complete(req, issue_flags, ret, 0);
 	return 0;
-- 
2.32.0

[PATCH 4/4] io_uring: accept directly into fixed file table

[Pavel Begunkov]

As done with open opcodes, allow accept to skip installing fd into
processes' file tables and put it directly into io_uring's fixed file
table. Same restrictions and design as for open.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index fd0dcee251b0..f7db43bf7dad 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4675,14 +4675,17 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 
 	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
 		return -EINVAL;
-	if (sqe->ioprio || sqe->len || sqe->buf_index)
+	if (sqe->ioprio || sqe->len)
 		return -EINVAL;
 
 	accept->addr = u64_to_user_ptr(READ_ONCE(sqe->addr));
 	accept->addr_len = u64_to_user_ptr(READ_ONCE(sqe->addr2));
 	accept->flags = READ_ONCE(sqe->accept_flags);
 	accept->nofile = rlimit(RLIMIT_NOFILE);
+	req->buf_index = READ_ONCE(sqe->file_index);
 
+	if (req->buf_index && (accept->flags & SOCK_CLOEXEC))
+		return -EINVAL;
 	if (accept->flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
 		return -EINVAL;
 	if (SOCK_NONBLOCK != O_NONBLOCK && (accept->flags & SOCK_NONBLOCK))
@@ -4695,28 +4698,34 @@ static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
 	struct io_accept *accept = &req->accept;
 	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
 	unsigned int file_flags = force_nonblock ? O_NONBLOCK : 0;
+	bool fixed = !!req->buf_index;
 	struct file *file;
 	int ret, fd;
 
 	if (req->file->f_flags & O_NONBLOCK)
 		req->flags |= REQ_F_NOWAIT;
 
-	fd = __get_unused_fd_flags(accept->flags, accept->nofile);
-	if (unlikely(fd < 0))
-		return fd;
-
+	if (!fixed) {
+		fd = __get_unused_fd_flags(accept->flags, accept->nofile);
+		if (unlikely(fd < 0))
+			return fd;
+	}
 	file = do_accept(req->file, file_flags, accept->addr, accept->addr_len,
 			 accept->flags);
 	if (IS_ERR(file)) {
+		if (!fixed)
+			put_unused_fd(fd);
 		ret = PTR_ERR(file);
 		if (ret == -EAGAIN && force_nonblock)
 			return -EAGAIN;
 		if (ret == -ERESTARTSYS)
 			ret = -EINTR;
 		req_set_fail(req);
-	} else {
+	} else if (!fixed) {
 		fd_install(fd, file);
 		ret = fd;
+	} else {
+		ret = io_install_fixed_file(req, file, issue_flags);
 	}
 	__io_req_complete(req, issue_flags, ret, 0);
 	return 0;
-- 
2.32.0

Re: [RFC 0/4] open/accept directly into io_uring fixed file table

[Jens Axboe]

On 7/7/21 5:39 AM, Pavel Begunkov wrote:
> Implement an old idea allowing open/accept io_uring requests to register
> a newly created file as a io_uring's fixed file instead of placing it
> into a task's file table. The switching is encoded in io_uring's SQEs
> by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
> think we need more, but may be a good idea to scrap u32 somewhere
> instead.
> 
> From the net side only needs a function doing __sys_accept4_file()
> but not installing fd, see 2/4.
> 
> Only RFC for now, the new functionality is tested only for open yet.
> I hope we can remember the author of the idea to add attribution.

Pretty sure the original suggester of this as Josh, CC'ed.

-- 
Jens Axboe

Re: [RFC 0/4] open/accept directly into io_uring fixed file table

[Stefan Metzmacher]

Am 07.07.21 um 15:07 schrieb Jens Axboe:
> On 7/7/21 5:39 AM, Pavel Begunkov wrote:
>> Implement an old idea allowing open/accept io_uring requests to register
>> a newly created file as a io_uring's fixed file instead of placing it
>> into a task's file table. The switching is encoded in io_uring's SQEs
>> by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
>> think we need more, but may be a good idea to scrap u32 somewhere
>> instead.
>>
>> From the net side only needs a function doing __sys_accept4_file()
>> but not installing fd, see 2/4.
>>
>> Only RFC for now, the new functionality is tested only for open yet.
>> I hope we can remember the author of the idea to add attribution.
> 
> Pretty sure the original suggester of this as Josh, CC'ed.

I also requested it for open :-)

metze

Re: [RFC 0/4] open/accept directly into io_uring fixed file table

[Jens Axboe]

On 7/7/21 7:59 AM, Stefan Metzmacher wrote:
> Am 07.07.21 um 15:07 schrieb Jens Axboe:
>> On 7/7/21 5:39 AM, Pavel Begunkov wrote:
>>> Implement an old idea allowing open/accept io_uring requests to register
>>> a newly created file as a io_uring's fixed file instead of placing it
>>> into a task's file table. The switching is encoded in io_uring's SQEs
>>> by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
>>> think we need more, but may be a good idea to scrap u32 somewhere
>>> instead.
>>>
>>> From the net side only needs a function doing __sys_accept4_file()
>>> but not installing fd, see 2/4.
>>>
>>> Only RFC for now, the new functionality is tested only for open yet.
>>> I hope we can remember the author of the idea to add attribution.
>>
>> Pretty sure the original suggester of this as Josh, CC'ed.
> 
> I also requested it for open :-)

Indeed! I honestly forget the details, as some of it is implementation
detail. I think Josh was the first to suggest a private fd table could
be used, but that's mostly implementation detail as the point was to be
able to know which fd would be assigned.

But I think we're all in agreement here, it's a nifty feature :-)

-- 
Jens Axboe

Re: [RFC 0/4] open/accept directly into io_uring fixed file table

[Josh Triplett]

On Wed, Jul 07, 2021 at 07:07:52AM -0600, Jens Axboe wrote:
> On 7/7/21 5:39 AM, Pavel Begunkov wrote:
> > Implement an old idea allowing open/accept io_uring requests to register
> > a newly created file as a io_uring's fixed file instead of placing it
> > into a task's file table. The switching is encoded in io_uring's SQEs
> > by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
> > think we need more, but may be a good idea to scrap u32 somewhere
> > instead.
> > 
> > From the net side only needs a function doing __sys_accept4_file()
> > but not installing fd, see 2/4.
> > 
> > Only RFC for now, the new functionality is tested only for open yet.
> > I hope we can remember the author of the idea to add attribution.
> 
> Pretty sure the original suggester of this as Josh, CC'ed.

Thanks for working on this, Pavel!

Original thread at
https://lore.kernel.org/io-uring/20200715004209.GA334456@localhost/T/ in
case that helps.

Re: [RFC 0/4] open/accept directly into io_uring fixed file table

[Pavel Begunkov]

On 7/7/21 4:04 PM, Josh Triplett wrote:
> On Wed, Jul 07, 2021 at 07:07:52AM -0600, Jens Axboe wrote:
>> On 7/7/21 5:39 AM, Pavel Begunkov wrote:
>>> Implement an old idea allowing open/accept io_uring requests to register
>>> a newly created file as a io_uring's fixed file instead of placing it
>>> into a task's file table. The switching is encoded in io_uring's SQEs
>>> by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
>>> think we need more, but may be a good idea to scrap u32 somewhere
>>> instead.
>>>
>>> From the net side only needs a function doing __sys_accept4_file()
>>> but not installing fd, see 2/4.
>>>
>>> Only RFC for now, the new functionality is tested only for open yet.
>>> I hope we can remember the author of the idea to add attribution.
>>
>> Pretty sure the original suggester of this as Josh, CC'ed.
> 
> Thanks for working on this, Pavel!
> 
> Original thread at

Totally was thinking it was only a discussion but not an actual patch,
and I even did comment on it! Sorry Josh, would have persuaded you to
finish it, if I remembered that.

> https://lore.kernel.org/io-uring/20200715004209.GA334456@localhost/T/ in
> case that helps.
> 

-- 
Pavel Begunkov

Re: [RFC 0/4] open/accept directly into io_uring fixed file table

[Hao Xu]

在 2021/7/7 下午7:39, Pavel Begunkov 写道:
> Implement an old idea allowing open/accept io_uring requests to register
> a newly created file as a io_uring's fixed file instead of placing it
> into a task's file table. The switching is encoded in io_uring's SQEs
> by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
> think we need more, but may be a good idea to scrap u32 somewhere
> instead.
> 
>  From the net side only needs a function doing __sys_accept4_file()
> but not installing fd, see 2/4.
> 
> Only RFC for now, the new functionality is tested only for open yet.
> I hope we can remember the author of the idea to add attribution.
> 
Great feature! I believe this one leverages linked sqes, we may need to
remind users to be careful when they use this feature in shared sqthread
mode since linked sqes may be splited.
> Pavel Begunkov (4):
>    io_uring: allow open directly into fixed fd table
>    net: add an accept helper not installing fd
>    io_uring: hand code io_accept()' fd installing
>    io_uring: accept directly into fixed file table
> 
>   fs/io_uring.c                 | 113 +++++++++++++++++++++++++++++-----
>   include/linux/socket.h        |   3 +
>   include/uapi/linux/io_uring.h |   2 +
>   net/socket.c                  |  71 +++++++++++----------
>   4 files changed, 138 insertions(+), 51 deletions(-)
>