Nouveau - Dec 2025 - [PATCH 10/11] gpu: nova-core: LibosMemoryRegionInitArgument size must be page aligned

On Wed, 2025-11-19 at 12:36 +0900, Alexandre Courbot
wrote:
> You can use the `Alignment` type here, as the rest of the code does:
> 
> ??? let size = num::usize_as_u64(obj.size())
> ??????? .align_up(Alignment::new::<GSP_PAGE_SIZE>())?;
> 
> Now `align_up` returns an error in case of overflow, that we will need
> to pass down to the caller by changing the return type of `new`. It is a
> bit annoying, but better than the behavior of `next_mutiple_of` in such
> a case, which is to panic. :)
I see your point, but these are u64s that we're talking about.  The only way
next_mutiple_of() can
panic is if obj.size() is greater than 0xFFFFFFFFFFFFF000, which is not
possible.

I would say in this case, a panic is preferable to a convoluted error return
that will never be
exercised, because failure here indicates a coding error, not an input error.

On Tue Dec 2, 2025 at 8:25 AM JST, Timur Tabi wrote:
> On Wed, 2025-11-19 at 12:36 +0900, Alexandre Courbot wrote:
>> You can use the `Alignment` type here, as the rest of the code does:
>> 
>> ??? let size = num::usize_as_u64(obj.size())
>> ??????? .align_up(Alignment::new::<GSP_PAGE_SIZE>())?;
>> 
>> Now `align_up` returns an error in case of overflow, that we will need
>> to pass down to the caller by changing the return type of `new`. It is
a
>> bit annoying, but better than the behavior of `next_mutiple_of` in such
>> a case, which is to panic. :)
>
> I see your point, but these are u64s that we're talking about.  The
only way next_mutiple_of() can
> panic is if obj.size() is greater than 0xFFFFFFFFFFFFF000, which is not
possible.
>
> I would say in this case, a panic is preferable to a convoluted error
return that will never be
> exercised, because failure here indicates a coding error, not an input
error.
The input data is a usize, so technically we could get an input that
triggers that error.

I know it's a very edge case, and clearly indicates a bug, but the
general rule is: don't panic the kernel. And in Rust, if possible, don't
even let me compiler insert panic-handling code. If you don't want to
change the return type of the method, then maybe use `unwrap_or` and
`inspect_err` to print an error before returning e.g. `0`.

But others have already thought "naah, that's never gonna happen"
and
got burnt very publicly [1], so let's learn from that. :P

[1] https://blog.cloudflare.com/18-november-2025-outage/