[PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512

[Christoph Lameter (Ampere)]

On Thu, 14 Mar 2024, Russell King (Oracle) wrote:

>> It is really surprising that this didn't blow up for anyone else so
>> far... This means that the $subject patch is fine.
>
> Wow. I guess we've been lucky with that allocation hitting memory
> containing zeros. Well done at tracking it down!

It would have blown up with slub_debug because that includes poisoning the 
contents of all allocations via the slab allocator. Why did that not 
occur? We should have seen a backtrace with data in registers etc showing 
poisoning values for an unitialized object.

Note that this was indeed triggered by OFFSTACK because 
(z)alloc_cpumask_var() only generates a kmalloc allocation if that option 
is set.

The config option was never set before my patch was applied on ARM64.