[RFC PATCH v3 00/18] io-uring/xfs: support async buffered writes

Stefan Roesch
2022-05-18T23:37:31Z

This patch series adds support for async buffered writes when using both
xfs and io-uring. Currently io-uring only supports buffered writes in the
slow path, by processing them in the io workers. With this patch series it is
now possible to support buffered writes in the fast path. To be able to use
the fast path the required pages must be in the page cache, the required locks
in xfs can be granted immediately and no additional blocks need to be read
form disk.

Updating the inode can take time. An optimization has been implemented for
the time update. Time updates will be processed in the slow path. While there
is already a time update in process, other write requests for the same file,
can skip the update of the modification time.

Performance results:
For fio the following results have been obtained with a queue depth of
1 and 4k block size (runtime 600 secs):

sequential writes:
without patch with patch libaio psync
iops: 77k 209k 195K 233K
bw: 314MB/s 854MB/s 790MB/s 953MB/s
clat: 9600ns 120ns 540ns 3000ns

For an io depth of 1, the new patch improves throughput by over three times
(compared to the exiting behavior, where buffered writes are processed by an
io-worker process) and also the latency is considerably reduced. To achieve the
same or better performance with the exisiting code an io depth of 4 is required.
Increasing the iodepth further does not lead to improvements.

In addition the latency of buffered write operations is reduced considerably.

Support for async buffered writes:

To support async buffered writes the flag FMODE_BUF_WASYNC is introduced. In
addition the check in generic_write_checks is modified to allow for async
buffered writes that have this flag set.

Changes to the iomap page create function to allow the caller to specify
the gfp flags. Sets the IOMAP_NOWAIT flag in iomap if IOCB_NOWAIT has been set
and specifies the requested gfp flags.

Adds the iomap async buffered write support to the xfs iomap layer.
Adds async buffered write support to the xfs iomap layer.

Support for async buffered write support and inode time modification

Splits the functions for checking if the file privileges need to be removed in
two functions: check function and a function for the removal of file privileges.
The same split is also done for the function to update the file modification time.

Implement an optimization that while a file modification time is pending other
requests for the same file don't need to wait for the file modification update.
This avoids that a considerable number of buffered async write requests get
punted.

Take the ilock in nowait mode if async buffered writes are enabled and enable
the async buffered writes optimization in io_uring.

Support for write throttling of async buffered writes:

Add a no_wait parameter to the exisiting balance_dirty_pages() function. The
function will return -EAGAIN if the parameter is true and write throttling is
required.

Add a new function called balance_dirty_pages_ratelimited_async() that will be
invoked from iomap_write_iter() if an async buffered write is requested.

Enable async buffered write support in xfs
This enables async buffered writes for xfs.

Testing:
This patch has been tested with xfstests and fio.

Changes:
V3:
- Reformat new code in generic_write_checks_count() to line lengthof 80.
- Remove if condition in __iomap_write_begin to maintain current behavior.
- use GFP_NOWAIT flag in __iomap_write_begin
- rename need_file_remove_privs() function to file_needs_remove_privs()
- rename do_file_remove_privs to __file_remove_privs()
- add kernel documentation to file_remove_privs() function
- rework else if branch in file_remove_privs() function
- add kernel documentation to file_modified() function
- add kernel documentation to file_modified_async() function
- rename err variable in file_update_time to ret
- rename function need_file_update_time() to file_needs_update_time()
- rename function do_file_update_time() to __file_update_time()
- don't move check for FMODE_NOCMTIME in generic helper
- reformat __file_update_time for easier reading
- add kernel documentation to file_update_time() function
- fix if in file_update_time from < to <=
- move modification of inode flags from do_file_update_time to file_modified()
When this function is called, the caller must hold the inode lock.
- 3 new patches from Jan to add new no_wait flag to balance_dirty_pages(),
remove patch 12 from previous series
- Make balance_dirty_pages_ratelimited_flags() a static function
- Add new balance_dirty_pages_ratelimited_async() function

V2:
- Remove atomic allocation
- Use direct write in xfs_buffered_write_iomap_begin()
- Use xfs_ilock_for_iomap() in xfs_buffered_write_iomap_begin()
- Remove no_wait check at the end of xfs_buffered_write_iomap_begin() for
the COW path.
- Pass xfs_inode pointer to xfs_ilock_iocb and rename function to
xfs_lock_xfs_inode
- Replace existing uses of xfs_ilock_iocb with xfs_ilock_xfs_inode
- Use xfs_ilock_xfs_inode in xfs_file_buffered_write()
- Callers of xfs_ilock_for_iomap need to initialize lock mode. This is
required so writes use an exclusive lock
- Split of _balance_dirty_pages() from balance_dirty_pages() and return
sleep time
- Call _balance_dirty_pages() in balance_dirty_pages_ratelimited_flags()
- Move call to balance_dirty_pages_ratelimited_flags() in iomap_write_iter()
to the beginning of the loop

Jan Kara (3):
mm: Move starting of background writeback into the main balancing loop
mm: Move updates of dirty_exceeded into one place
mm: Prepare balance_dirty_pages() for async buffered writes

Stefan Roesch (15):
block: Add check for async buffered writes to generic_write_checks
iomap: Add iomap_page_create_gfp to allocate iomap_pages
iomap: Use iomap_page_create_gfp() in __iomap_write_begin
iomap: Add async buffered write support
xfs: Add iomap async buffered write support
fs: Split off remove_needs_file_privs() __remove_file_privs()
fs: Split off file_needs_update_time and __file_update_time
xfs: Enable async write file modification handling.
fs: Optimization for concurrent file time updates.
xfs: Add async buffered write support
io_uring: Add support for async buffered writes
mm: Add balance_dirty_pages_ratelimited_async() function
iomap: Use balance_dirty_pages_ratelimited_flags in iomap_write_iter
io_uring: Add tracepoint for short writes
xfs: Enable async buffered write support

fs/inode.c | 178 ++++++++++++++++++++++++--------
fs/io_uring.c | 32 +++++-
fs/iomap/buffered-io.c | 71 +++++++++++--
fs/read_write.c | 4 +-
fs/xfs/xfs_file.c | 36 +++----
fs/xfs/xfs_iomap.c | 14 ++-
include/linux/fs.h | 7 ++
include/linux/writeback.h | 1 +
include/trace/events/io_uring.h | 25 +++++
mm/page-writeback.c | 108 ++++++++++++-------
10 files changed, 352 insertions(+), 124 deletions(-)

base-commit: 0cdd776ec92c0fec768c7079331804d3e52d4b27
--
2.30.2