Do we need an opt-in for file systems use of hw atomic writes?

[Christoph Hellwig]

On Tue, Aug 19, 2025 at 12:42:01PM +0100, John Garry wrote:
> nothing has been happening on this thread for a while. I figure that it is 
> because we have no good or obvious options.
>
> I think that it's better deal with the NVMe driver handling of AWUPF first, 
> as this applies to block fops as well.
>
> As for the suggestion to have an opt-in to use AWUPF, you wrote above that 
> users may not know when to enable this opt-in or not.
>
> It seems to me that we can give the option, but clearly label that it is 
> potentially dangerous. Hopefully the $RANDOMUSER with the $CHEAPO SSD will 
> be wise and steer clear.
>
> If we always ignore AWUPF, I fear that lots of sound NVMe implementations 
> will be excluded from HW atomics.

I think ignoring AWUPF is a good idea, but I've also hard some folks
not liking that.

The reason why I prefer a mount option is because we add that to fstab
and the kernel command line easily.  For block layer or driver options
we'd either need a sysfs file which is always annoying to apply at boot
time, or a module option which has the downside of applying to all
devices.