Re: [PATCH 1/3] memory: extern memory_block_size_bytes and set_memory_block_size_order

[David Hildenbrand]

On 08.10.24 17:21, Gregory Price wrote:
> On Tue, Oct 08, 2024 at 05:02:33PM +0200, David Hildenbrand wrote:
> > On 08.10.24 16:51, Gregory Price wrote:
> > > > > +int __weak set_memory_block_size_order(unsigned int order)
> > > > > +{
> > > > > +	return -ENODEV;
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(set_memory_block_size_order);
> > > >
> > > > I can understand what you are trying to achieve, but letting arbitrary
> > > > modules mess with this sounds like a bad idea.
> > >
> > > I suppose the alternative is trying to scan the CEDT from inside each
> > > machine, rather than the ACPI driver?  Seems less maintainable.
> > >
> > > I don't entirely disagree with your comment.  I hummed and hawwed over
> > > externing this - hence the warning in the x86 machine.
> > >
> > > Open to better answers.
> >
> > Maybe an interface to add more restrictions on the maximum size might be
> > better (instead of setting the size/order, you would impose another upper
> > limit).
>
> That is effectively what set_memory_block_size_order is, though.  Once
> blocks are exposed to the allocators, its no longer safe to change the
> size (in part because it was built assuming it wouldn't change, but I
> imagine there are other dragons waiting in the shadows to bite me).

Yes, we must run very early.

How is this supposed to interact with code like

set_block_size()

that also calls set_memory_block_size_order() on UV systems (assuming 
there will be CXL support sooner or later?)?


> So this would basically amount to a lock-bit being set in the architecture,
> beyond which block size can no longer be changed and a big ol' splat
> can be generated that says "NO TOUCH".
>
> > Just imagine having various users of such an interface ..
>
> I don't wanna D:

Right, and it also doesn't make sense as explained in my other comment: 
this should never apply to loaded modules. :)

--
Cheers,

David / dhildenb