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

[John Garry]

On 19/08/2025 14:39, Christoph Hellwig wrote:
> 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.

Disabling reading AWUPF would be the best way to know that for sure :)

> 
> 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, 

Could system-udev auto enable for us via sysfs file or ioctl?

> or a module option which has the downside of applying to all
> devices.

About the mount option, I suppose that it won't do much harm - it's just 
a bit of extra work to configure.

I just fear that admins will miss enabling it or not enable it out of 
doubt and users won't see the benefit of HW atomics.