linux-kernel - Re: [PATCH 1/3] memory: extern memory_block_size_bytes and set_memory_block_size

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Message-ID: <ZwVOE6JRS8Fd9_a8@PC2K9PVX.TheFacebook.com>
Date: Tue, 8 Oct 2024 11:21:55 -0400
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: linux-cxl@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
	osalvador@...e.de, gregkh@...uxfoundation.org, rafael@...nel.org,
	akpm@...ux-foundation.org, dan.j.williams@...el.com,
	Jonathan.Cameron@...wei.com, alison.schofield@...el.com,
	rrichter@....com, terry.bowman@....com, lenb@...nel.org,
	dave.jiang@...el.com, ira.weiny@...el.com
Subject: Re: [PATCH 1/3] memory: extern memory_block_size_bytes and
 set_memory_block_size_order

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).

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:

> 
> -- 
> Cheers,
> 
> David / dhildenb
>