re-enable IOCB_NOWAIT writes to files v2

[Christian Brauner]

On Thu, Nov 20, 2025 at 07:47:21AM +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.

It's a bit too close to the merge window for my taste and we have about
17 pull request topics for this cycle already.

So I'll take this for vfs-6.20.iomap. As usual I'll create that branch
now so that the patches don't get lost and will rebase once v6.19-rc1 is
out.