[RFC 0/7] io_uring lseek

Hao Xu
2023-07-26T10:26:17Z

From: Hao Xu <howeyxu@tencent.com>

This series adds lseek for io_uring, the motivation to import this
syscall is in previous io_uring getdents patchset, we lack a way to
rewind the file cursor when it goes to the end of file. Another reason
is lseek is a common syscall, it's good for coding consistency when
users use io_uring as their main loop.

Patch 1 is code clean for iomap
Patch 2 adds IOMAP_NOWAIT logic for iomap lseek
Patch 3 adds a nowait parameter to for IOMAP_NOWAIT control
Patch 4 adds llseek_nowait() for file_operations so that specific
filesystem can implement it for nowait lseek
Patch 5 adds llseek_nowait() implementation for xfs
Patch 6 adds a new vfs wrapper for io_uring use
Patch 7 is the main io_uring lseek implementation

Note, this series depends on the previous io_uring getdents series.

This is marked RFC since there is (at least) an issue to be discussed:
The work in this series is mainly to reslove a problem that the current
llseek() in struct file_operations doesn't have a place to deliver
nowait info, and adding an argument to it results in update for llseek
implementation of all filesystems (35 functions), so here I introduce
a new llseek_nowait() as a workaround.

For performance, it has about 20%~30% improvement on iops.
The test program is just like the one for io_uring getdents, here is the
link to it: https://github.com/HowHsu/liburing/blob/llseek/test/lseek.c
- Each test runs about 30000 async requests/sync syscalls
- Each test runs 100 times and get the average value.
- offset is randomly generated value
- the file is a 1M all zero file

[howeyxu@~]$ python3 run_lseek.py
test args: seek mode:SEEK_SET, offset: 334772
Average of sync : 0.012300650000000002
Average of iouring : 0.008528009999999999
30.67%

[howeyxu@~]$ python3 run_lseek.py
test args: seek mode:SEEK_CUR, offset: 389292
Average of sync : 0.012736129999999995
Average of iouring : 0.00928725
27.08%

[howeyxu@~]$ python3 run_lseek.py
test args: seek mode:SEEK_END, offset: 281141
Average of sync : 0.01221595
Average of iouring : 0.008442890000000003
30.89%

[howeyxu@~]$ python3 run_lseek.py
test args: seek mode:SEEK_DATA, offset: 931103
Average of sync : 0.015496230000000005
Average of iouring : 0.012341509999999998
20.36%

[howeyxu@~]$ python3 run_lseek.py
test args: seek mode:SEEK_HOLE, offset: 430194
Average of sync : 0.01555663000000001
Average of iouring : 0.012064940000000003
22.45%

Hao Xu (7):
iomap: merge iomap_seek_hole() and iomap_seek_data()
xfs: add nowait support for xfs_seek_iomap_begin()
add nowait parameter for iomap_seek()
add llseek_nowait() for struct file_operations
add llseek_nowait support for xfs
add vfs_lseek_nowait()
add lseek for io_uring

fs/ext4/file.c | 9 ++---
fs/gfs2/inode.c | 4 +--
fs/iomap/seek.c | 42 ++++++-----------------
fs/read_write.c | 18 ++++++++++
fs/xfs/xfs_file.c | 34 ++++++++++++++++---
fs/xfs/xfs_iomap.c | 4 ++-
include/linux/fs.h | 4 +++
include/linux/iomap.h | 6 ++--
include/uapi/linux/io_uring.h | 1 +
io_uring/fs.c | 63 +++++++++++++++++++++++++++++++++++
io_uring/fs.h | 3 ++
io_uring/opdef.c | 8 +++++
12 files changed, 145 insertions(+), 51 deletions(-)

base-commit: 4a4b046082eca8ae90b654d772fccc30e9f23f4d
--
2.25.1