re-enable IOCB_NOWAIT writes to files v4

Christoph Hellwig
2025-12-23T00:38:07Z

Hi all,

commit 66fa3cedf16a ("fs: Add async write file modification handling.")
effectively disabled IOCB_NOWAIT writes as timestamp updates currently
always require blocking, and the modern timestamp resolution means we
always update timestamps. This leads to a lot of context switches from
applications using io_uring to submit file writes, making it often worse
than using the legacy aio code that is not using IOCB_NOWAIT.

This series allows non-blocking updates for lazytime if the file system
supports it, and adds that support for XFS.

Changes since v3:
- fix was_dirty_time handling in __mark_inode_dirty for the racy flag
update case
- refactor inode_update_timestamps to make the lazytime vs blocking
logical more clear
- allow non-blocking timestamp updates for fat

Changes since v2:
- drop patches merged upstream
- adjust for the inode state accesors
- keep a check in __writeback_single_inode instead of exercising
potentially undefined behavior
- more spelling fixes

Changes since v1:
- more regular numbering of the S_* flags
- fix XFS to actually not block
- don't ignore the generic_update_time return value in
file_update_time_flags
- fix the sync_lazytime return value
- fix an out of data comment in btrfs
- fix a race that would update i_version before returning -EAGAIN in XFS

Diffstat:
Documentation/filesystems/locking.rst | 2
Documentation/filesystems/vfs.rst | 6 +
fs/btrfs/inode.c | 8 +-
fs/fs-writeback.c | 33 +++++++---
fs/gfs2/inode.c | 6 +
fs/inode.c | 111 +++++++++++++++++++++-------------
fs/internal.h | 3
fs/nfs/inode.c | 4 -
fs/orangefs/inode.c | 5 +
fs/overlayfs/inode.c | 2
fs/sync.c | 4 -
fs/ubifs/file.c | 13 ++-
fs/xfs/xfs_iops.c | 34 +++++++++-
fs/xfs/xfs_super.c | 29 --------
include/linux/fs.h | 27 ++++++--
include/trace/events/writeback.h | 6 -
16 files changed, 182 insertions(+), 111 deletions(-)