RE: [PATCH v4 2/2] io_uring: add support for IORING_OP_GETDENTS

RE: [PATCH v4 2/2] io_uring: add support for IORING_OP_GETDENTS

[Tavian Barnes]

 > IORING_OP_GETDENTS behaves much like getdents64(2) and takes the same
 > arguments, but with a small twist: it takes an additional offset
 > argument, and reading from the specified directory starts at the given
 > offset.
 >
 > For the first IORING_OP_GETDENTS call on a directory, the offset
 > parameter can be set to zero, and for subsequent calls, it can be
 > set to the ->d_off field of the last struct linux_dirent64 returned
 > by the previous IORING_OP_GETDENTS call.
 >
 > Internally, if necessary, IORING_OP_GETDENTS will vfs_llseek() to
 > the right directory position before calling vfs_getdents().

For my envisioned use case it would be important to support reading from
the current file position when offset == -1 (IORING_FEAT_RW_CUR_POS).
Among other things, this would let me fully read a directory with fewer
round-trips.  Normally after the first getdents(), another one must be
issued to distinguish between EOF and a short first read.  It would be
nice to do both calls with linked SQEs so I could immediately know that
I've reached the end of the directory.

 > IORING_OP_GETDENTS may or may not update the specified directory's
 > file offset, and the file offset should not be relied upon having
 > any particular value during or after an IORING_OP_GETDENTS call.

Obviously for the above to work, we'd have to get rid of this 
limitation.  Is that possible?

Re: [PATCH v4 2/2] io_uring: add support for IORING_OP_GETDENTS

[Lennert Buytenhek]

Hello!


On Tue, Mar 16, 2021 at 11:39:25AM -0400, Tavian Barnes wrote:

> > IORING_OP_GETDENTS behaves much like getdents64(2) and takes the same
> > arguments, but with a small twist: it takes an additional offset
> > argument, and reading from the specified directory starts at the given
> > offset.
> >
> > For the first IORING_OP_GETDENTS call on a directory, the offset
> > parameter can be set to zero, and for subsequent calls, it can be
> > set to the ->d_off field of the last struct linux_dirent64 returned
> > by the previous IORING_OP_GETDENTS call.
> >
> > Internally, if necessary, IORING_OP_GETDENTS will vfs_llseek() to
> > the right directory position before calling vfs_getdents().
> 
> For my envisioned use case it would be important to support reading from
> the current file position when offset == -1 (IORING_FEAT_RW_CUR_POS).

Thank you for the feedback!  This is easy enough to do:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 306e2bd9fd75..1ffe58462ab3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4349,7 +4349,7 @@ static int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
 	/* for vfs_llseek and to serialize ->iterate_shared() on this file */
 	mutex_lock(&req->file->f_pos_lock);
 
-	if (req->file->f_pos != getdents->pos) {
+	if (getdents->pos != -1 && req->file->f_pos != getdents->pos) {
 		loff_t res = vfs_llseek(req->file, getdents->pos, SEEK_SET);
 		if (res < 0)
 			ret = res;

(A corresponding patch to the liburing testsuite is below.)


> Among other things, this would let me fully read a directory with fewer
> round-trips.  Normally after the first getdents(), another one must be
> issued to distinguish between EOF and a short first read.  It would be
> nice to do both calls with linked SQEs so I could immediately know that
> I've reached the end of the directory.

Does that work with this patch?


> > IORING_OP_GETDENTS may or may not update the specified directory's
> > file offset, and the file offset should not be relied upon having
> > any particular value during or after an IORING_OP_GETDENTS call.
> 
> Obviously for the above to work, we'd have to get rid of this
> limitation.  Is that possible?

The idea behind wording it this way was that this would allow for
further in-kernel parallelization of getdents calls in the future
(by not serializing on the current-file-position-mutex), but I can
see how this behavior conflicts with your use case.

Also, given how the current implementation _does_ update the
directory's file offset, some userspace code will surely end up
depending on that behavior, which would make it hard to change
in the future.

Also, allowing unserialized in-kernel getdents calls on the same file
descriptor would require a bunch more VFS work (including auditing
all filesystems), and, to be completely honest, I am unlikely to be
doing that work.

All in all, maybe we should just state that IORING_OP_GETDENTS is
a seek (if needed) plus a getdents combo that does together update
the directory's file offset, and be done with it.




A corresponding incremental patch to the testsuite to test the
"off == -1 means current file position" case:

diff --git a/test/getdents.c b/test/getdents.c
index 3ca7b05..532e6f4 100644
--- a/test/getdents.c
+++ b/test/getdents.c
@@ -130,6 +130,7 @@ int main(int argc, char *argv[])
 	while (bufp < end) {
 		struct linux_dirent64 *dent;
 		uint8_t buf2[BUFSZ];
+		off_t ret2;
 
 		dent = (struct linux_dirent64 *)bufp;
 
@@ -165,6 +166,47 @@ int main(int argc, char *argv[])
 
 			return 1;
 		}
+
+		/*
+		 * Now seek the directory to the given offset manually,
+		 * and perform a getdents call with offset -1 (which means:
+		 * "read from current offset"), and make sure we get the
+		 * same data back.
+		 */
+		ret2 = lseek(dirfd, dent->d_off, SEEK_SET);
+		if (ret2 != dent->d_off) {
+			if (ret2 == (off_t)-1) {
+				perror("lseek");
+			} else {
+				fprintf(stderr, "error seeking directory "
+						"to offset %" PRId64 "\n",
+					(uint64_t)dent->d_off);
+			}
+			return 1;
+		}
+
+		memset(buf2, 0, sizeof(buf2));
+
+		ret = test_getdents(&ring, buf2, sizeof(buf2), (uint64_t)-1);
+		if (ret < 0) {
+			fprintf(stderr, "getdents: %s\n", strerror(-ret));
+			return 1;
+		}
+
+		if (ret != end - bufp || memcmp(bufp, buf2, ret)) {
+			fprintf(stderr, "getdents: read from seeked offset "
+					"%" PRId64 " returned unexpected "
+					"data\n\n", (uint64_t)dent->d_off);
+
+			fprintf(stderr, "read from offset zero:\n");
+			dump_dents(bufp, end - bufp);
+
+			fprintf(stderr, "offsetted read:\n");
+			dump_dents(buf2, ret);
+
+			return 1;
+		}
+
 	}
 
 	if (!found_dot)

[PATCH v4 0/2] io_uring: add support for IORING_OP_GETDENTS

[Lennert Buytenhek]

(These patches depend on IORING_OP_MKDIRAT going in first -- see
the changelog entry for v2 below.)

These patches add support for IORING_OP_GETDENTS, which is a new io_uring
opcode that more or less does an lseek(sqe->fd, sqe->off, SEEK_SET)
followed by a getdents64(sqe->fd, (void *)sqe->addr, sqe->len).

A dumb test program which recursively scans through a directory tree
and prints the names of all directories and files it encounters along
the way is available here:

	https://krautbox.wantstofly.org/~buytenh/uringfind-v3.c

Changes since v3 RFC:

- Made locking in io_getdents() unconditional, as the prior
  optimization was racy.  (Pointed out by Pavel Begunkov.)

- Rebase onto for-5.13/io_uring as of 2021/03/12 plus a manually
  applied version of the mkdirat patch.

Changes since v2 RFC:

- Rebase onto io_uring-2021-02-17 plus a manually applied version of
  the mkdirat patch.  The latter is needed because userland (liburing)
  has already merged the opcode for IORING_OP_MKDIRAT (in commit
  "io_uring.h: 5.12 pending kernel sync") while this opcode isn't in
  the kernel yet (as of io_uring-2021-02-17), and this means that this
  can't be merged until IORING_OP_MKDIRAT is merged.

- Adapt to changes made in "io_uring: replace force_nonblock with flags"
  that are in io_uring-2021-02-17.

Changes since v1 RFC:

- Drop the trailing '64' from IORING_OP_GETDENTS64 (suggested by
  Matthew Wilcox).

- Instead of requiring that sqe->off be zero, use this field to pass
  in a directory offset to start reading from.  For the first
  IORING_OP_GETDENTS call on a directory, this can be set to zero,
  and for subsequent calls, it can be set to the ->d_off field of
  the last struct linux_dirent64 returned by the previous call.

Lennert Buytenhek (2):
  readdir: split the core of getdents64(2) out into vfs_getdents()
  io_uring: add support for IORING_OP_GETDENTS

 fs/io_uring.c                 |   66 ++++++++++++++++++++++++++++++++++++++++++
 fs/readdir.c                  |   25 ++++++++++-----
 include/linux/fs.h            |    4 ++
 include/uapi/linux/io_uring.h |    1
 4 files changed, 88 insertions(+), 8 deletions(-)

[PATCH v4 2/2] io_uring: add support for IORING_OP_GETDENTS

[Lennert Buytenhek]

IORING_OP_GETDENTS behaves much like getdents64(2) and takes the same
arguments, but with a small twist: it takes an additional offset
argument, and reading from the specified directory starts at the given
offset.

For the first IORING_OP_GETDENTS call on a directory, the offset
parameter can be set to zero, and for subsequent calls, it can be
set to the ->d_off field of the last struct linux_dirent64 returned
by the previous IORING_OP_GETDENTS call.

Internally, if necessary, IORING_OP_GETDENTS will vfs_llseek() to
the right directory position before calling vfs_getdents().

IORING_OP_GETDENTS may or may not update the specified directory's
file offset, and the file offset should not be relied upon having
any particular value during or after an IORING_OP_GETDENTS call.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
---
 fs/io_uring.c                 | 66 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/io_uring.h |  1 +
 2 files changed, 67 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index eef957139915..306e2bd9fd75 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -640,6 +640,13 @@ struct io_mkdir {
 	struct filename			*filename;
 };
 
+struct io_getdents {
+	struct file			*file;
+	struct linux_dirent64 __user	*dirent;
+	unsigned int			count;
+	loff_t				pos;
+};
+
 struct io_completion {
 	struct file			*file;
 	struct list_head		list;
@@ -774,6 +781,7 @@ struct io_kiocb {
 		struct io_rename	rename;
 		struct io_unlink	unlink;
 		struct io_mkdir		mkdir;
+		struct io_getdents	getdents;
 		/* use only after cleaning per-op data, see io_clean_op() */
 		struct io_completion	compl;
 	};
@@ -988,6 +996,9 @@ static const struct io_op_def io_op_defs[] = {
 	[IORING_OP_RENAMEAT] = {},
 	[IORING_OP_UNLINKAT] = {},
 	[IORING_OP_MKDIRAT] = {},
+	[IORING_OP_GETDENTS] = {
+		.needs_file		= 1,
+	},
 };
 
 static bool io_disarm_next(struct io_kiocb *req);
@@ -4310,6 +4321,56 @@ static int io_sync_file_range(struct io_kiocb *req, unsigned int issue_flags)
 	return 0;
 }
 
+static int io_getdents_prep(struct io_kiocb *req,
+			    const struct io_uring_sqe *sqe)
+{
+	struct io_getdents *getdents = &req->getdents;
+
+	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
+		return -EINVAL;
+	if (sqe->ioprio || sqe->rw_flags || sqe->buf_index)
+		return -EINVAL;
+
+	getdents->pos = READ_ONCE(sqe->off);
+	getdents->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr));
+	getdents->count = READ_ONCE(sqe->len);
+	return 0;
+}
+
+static int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_getdents *getdents = &req->getdents;
+	int ret = 0;
+
+	/* getdents always requires a blocking context */
+	if (issue_flags & IO_URING_F_NONBLOCK)
+		return -EAGAIN;
+
+	/* for vfs_llseek and to serialize ->iterate_shared() on this file */
+	mutex_lock(&req->file->f_pos_lock);
+
+	if (req->file->f_pos != getdents->pos) {
+		loff_t res = vfs_llseek(req->file, getdents->pos, SEEK_SET);
+		if (res < 0)
+			ret = res;
+	}
+
+	if (ret == 0) {
+		ret = vfs_getdents(req->file, getdents->dirent,
+				   getdents->count);
+	}
+
+	mutex_unlock(&req->file->f_pos_lock);
+
+	if (ret < 0) {
+		if (ret == -ERESTARTSYS)
+			ret = -EINTR;
+		req_set_fail_links(req);
+	}
+	io_req_complete(req, ret);
+	return 0;
+}
+
 #if defined(CONFIG_NET)
 static int io_setup_async_msg(struct io_kiocb *req,
 			      struct io_async_msghdr *kmsg)
@@ -5813,6 +5874,8 @@ static int io_req_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return io_unlinkat_prep(req, sqe);
 	case IORING_OP_MKDIRAT:
 		return io_mkdirat_prep(req, sqe);
+	case IORING_OP_GETDENTS:
+		return io_getdents_prep(req, sqe);
 	}
 
 	printk_once(KERN_WARNING "io_uring: unhandled opcode %d\n",
@@ -6075,6 +6138,9 @@ static int io_issue_sqe(struct io_kiocb *req, unsigned int issue_flags)
 	case IORING_OP_MKDIRAT:
 		ret = io_mkdirat(req, issue_flags);
 		break;
+	case IORING_OP_GETDENTS:
+		ret = io_getdents(req, issue_flags);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 89b1225998c0..b12d49361022 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -138,6 +138,7 @@ enum {
 	IORING_OP_RENAMEAT,
 	IORING_OP_UNLINKAT,
 	IORING_OP_MKDIRAT,
+	IORING_OP_GETDENTS,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
-- 
2.29.2