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

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Message-ID: <670582254a5db_2e172294fe@iweiny-mobl.notmuch>
Date: Tue, 8 Oct 2024 14:04:05 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Gregory Price <gourry@...rry.net>, 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

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).

Yea I think this is along the idea I had.  But much clearer.

Ira

> 
> 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
> >