Re: re-enable IOCB_NOWAIT writes to files

Jeff Layton
2025-11-14T14:05:01Z

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@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?