[Pkg-zfsonlinux-devel] Bug#1105740: Bug#1105740: arc_summary data target values nonsensical

[Shengqi Chen]

Hi,

> 2025年5月14日 11:13，Michael reports bugs <bugs at deegan.id.au> 写道：
> 
>  MFU data target:  -373596196279.3 %  -27644722171524358144 Bytes
> 
>  MRU data target:                     373596196366.3 %   24.0 EiB
> 
> I think these are wrong. I'm guessing some sort of signed arithmetic bug.

arcstat is a python script [1], so it should have singed conversion problems.

From [1] we can see the calculation:

* s = 4294967296 = 2^32
* MFU data target: v = (s-int(pd))*(s-int(meta))/s
* MRU data target: v = int(pd)*(s-int(meta))/s
* MRU metadata target: v = int(pm)*int(meta)/s

Since MRU metadata target looks normal, meta and s should all be good.
By some calculation we know pd ~= -1.60e19, which is close to ~2^63.8.
And this number is read directly from /proc/spl/kstat/zfs.

I did not have time to investigate further, but I am sure this is some
issue from the zfs kmod statistics (maybe still signed / unsigned).
Does this problem reproduces after a reboot?

[1]: https://github.com/openzfs/zfs/blob/10a78e26479325a40ace80198249a36906b4d446/cmd/arc_summary#L629

Thanks,
Shengqi Chen