Re: [PATCH] fs: Propagate FMODE_NOCMTIME flag to user-facing O_NOCMTIME

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 06, 2025 at 09:06:40AM +1100, Dave Chinner wrote:
> If you don't care about accurate c/mtime, then mount the filesystem
> with '-o lazytime' to degrade c/mtime updates to "eventual
> consistency" behaviour for IO operations.

Exactly.

> Lazytime updates can generally be done in a non-blocking manner
> right now (someone raised that in the context of io-uring on #xfs
> about a month ago), but the NOWAIT behaviour for timestamp updates
> is done at a higher level in the VFS and does not take into account
> filesystem specific non-blocking lazytime updates at all.  If we
> push the NOWAIT checking behaviour down to the filesystem, we can do
> this.

We might not even have to push it out, but just make the VFS/rw helper
check aware of lazytime.  Either way currently even a lazytime
timestampt update will cause a write to block, which renders the
nowait writes pretty useless on anything but block devices.