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

[David Hildenbrand]

On 14.10.24 16:25, Gregory Price wrote:
> On Mon, Oct 14, 2024 at 01:54:27PM +0200, David Hildenbrand wrote:
> > 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?)?
>
> Tying the other email to this one - just clarifying the way forward here.
>
> It sounds like you're saying at a minimum drop EXPORT tags to prevent
> modules from calling it - but it also sounds like built-ins need to be
> prevented from touching it as well after a certain point in early boot.

Right, at least the EXPORT is not required.

> Do you think I should go down the advise() path as suggested by Ira,
> just adding a arch_lock_blocksize() bit and have set_..._order check it,
> or should we just move towards each architecture having to go through
> the ACPI:CEDT itself?

Let's summarize what we currently have on x86 is:

1) probe_memory_block_size()

Triggered on first memory_block_size_bytes() invocation. Makes a 
decision based on:

a) Already set size using set_memory_block_size_order()
b) RAM size
c) Bare metal vs. virt (bare metal -> use max)
d) Virt: largest block size aligned to memory end


2) set_memory_block_size_order()

Triggered by set_block_size() on UV systems.


I don't think set_memory_block_size_order() is the right tool to use. We 
just want to leave that alone I think -- it's a direct translation of a 
kernel cmdline parameter that should win.

You essentially want to tweak the b)->d) logic to take other alignment 
into consideration.

Maybe have some simple callback mechanism probe_memory_block_size() that 
can consult other sources for alignment requirements?

If that's not an option, then another way to set further min-alignment 
requirements (whereby we take MIN(old_align, new_align))?

--
Cheers,

David / dhildenb