re-enable IOCB_NOWAIT writes to files v4

[Christoph Hellwig]

Julia found a nice little typo in the new changes, which really
wants me to split the atime from c/mtime update path badly.  So
don't feel rushed to review this version and enjoy the holidays,
there will be a new one soon.

On Tue, Dec 23, 2025 at 09:37:43AM +0900, Christoph Hellwig wrote:
> 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(-)
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