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

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Message-ID: <Zw0p7-7ymn-0bPiC@PC2K9PVX.TheFacebook.com>
Date: Mon, 14 Oct 2024 10:25:51 -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 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.

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?

Doesn't sound like we've quite hit a consensus on where the actual
adjustment logic should land - just that this shouldn't be touched by modules.

~Gregory