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Subject[PATCH v3 0/2] io_uring: add support for IORING_OP_GETDENTS
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 for IORING_OP_GETDENTS is available here:

https://krautbox.wantstofly.org/~buytenh/uringfind-v2.c

This test program does something along the lines of what find(1) does:
it scans recursively through a directory tree and prints the names of
all directories and files it encounters along the way -- but then using
io_uring. (The io_uring version prints the names of encountered files and
directories in an order that's determined by SQE completion order, which
is somewhat nondeterministic and likely to differ between runs.)

On a directory tree with 14-odd million files in it that's on a
six-drive (spinning disk) btrfs raid, find(1) takes:

# echo 3 > /proc/sys/vm/drop_caches
# time find /mnt/repo > /dev/null

real 24m7.815s
user 0m15.015s
sys 0m48.340s
#

And the io_uring version takes:

# echo 3 > /proc/sys/vm/drop_caches
# time ./uringfind /mnt/repo > /dev/null

real 10m29.064s
user 0m4.347s
sys 0m1.677s
#


The fully cached case also shows some speedup. find(1):

# time find /mnt/repo > /dev/null

real 0m5.223s
user 0m1.926s
sys 0m3.268s
#

Versus the io_uring version:

# time ./uringfind /mnt/repo > /dev/null

real 0m3.604s
user 0m2.417s
sys 0m0.793s
#


That said, the point of this patch isn't primarily to enable
lightning-fast find(1) or du(1), but more to complete the set of
filesystem I/O primitives available via io_uring, so that applications
can do all of their filesystem I/O using the same mechanism, without
having to manually punt some of their work out to worker threads -- and
indeed, an object storage backend server that I wrote a while ago can
run with a pure io_uring based event loop with this 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 | 73 ++++++++++++++++++++++++++++++++++++++++++
fs/readdir.c | 25 +++++++++-----
include/linux/fs.h | 4 ++
include/uapi/linux/io_uring.h | 1
4 files changed, 95 insertions(+), 8 deletions(-)

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