re-enable IOCB_NOWAIT writes to files

[Jeff Layton]

On Fri, 2025-11-14 at 07:26 +0100, 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.
> 
> It also fixes the layering bypass in btrfs when updating timestamps on
> device files for devices removed from btrfs usage, and FMODE_NOCMTIME
> handling in the VFS now that nfsd started using it.  Note that I'm still
> not sure that nfsd usage is fully correct for all file systems, as only
> XFS explicitly supports FMODE_NOCMTIME, but at least the generic code
> does the right thing now.
> 
> Diffstat:
>  Documentation/filesystems/locking.rst |    2 
>  Documentation/filesystems/vfs.rst     |    6 ++
>  fs/btrfs/inode.c                      |    3 +
>  fs/btrfs/volumes.c                    |   11 +--
>  fs/fat/misc.c                         |    3 +
>  fs/fs-writeback.c                     |   53 ++++++++++++++----
>  fs/gfs2/inode.c                       |    6 +-
>  fs/inode.c                            |  100 +++++++++++-----------------------
>  fs/internal.h                         |    3 -
>  fs/orangefs/inode.c                   |    7 ++
>  fs/overlayfs/inode.c                  |    3 +
>  fs/sync.c                             |    4 -
>  fs/ubifs/file.c                       |    9 +--
>  fs/utimes.c                           |    1 
>  fs/xfs/xfs_iops.c                     |   29 ++++++++-
>  fs/xfs/xfs_super.c                    |   29 ---------
>  include/linux/fs.h                    |   17 +++--
>  include/trace/events/writeback.h      |    6 --
>  18 files changed, 152 insertions(+), 140 deletions(-)

This all looks pretty reasonable to me. There are a few changelog and
subject line typos, but the code changes look fine. You can add:

Reviewed-by: Jeff Layton <jlayton at kernel.org>

As far as nfsd's usage of FMODE_NOCMTIME, it looks OK to me. That's
implemented today by the check in file_modified_flags(), which is
generic and should work across filesystems.

The main exception is xfs_exchange_range() which has some special
handling for it, but nfsd doesn't use that functionality so that
shouldn't be an issue.

Am I missing some subtlety?