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

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Message-ID: <ZwVG8Z3GRYLoL_Jk@PC2K9PVX.TheFacebook.com>
Date: Tue, 8 Oct 2024 10:51:29 -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 04:03:37PM +0200, David Hildenbrand wrote:
> On 08.10.24 06:43, Gregory Price wrote:
> > On CXL systems, block alignment may be as small as 256MB, which may
> > require a resize of the block size during initialization.  This is done
> > in the ACPI driver, so the resize function need to be made available.
> > 
> > Presently, only x86 provides the functionality to resize memory
> > block sizes.  Wire up a weak stub for set_memory_block_size_order,
> > similar to memory_block_size_bytes, that simply returns -ENODEV.
> > 
> > Since set_memory_block_size_order is now extern, we also need to
> > drop the __init macro.
> > 
> > Signed-off-by: Gregory Price <gourry@...rry.net>
> > ---
> >   arch/x86/mm/init_64.c  | 2 +-
> >   drivers/base/memory.c  | 6 ++++++
> >   include/linux/memory.h | 4 ++--
> >   3 files changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> > index ff253648706f..6086f99449fa 100644
> > --- a/arch/x86/mm/init_64.c
> > +++ b/arch/x86/mm/init_64.c
> > @@ -1424,7 +1424,7 @@ void mark_rodata_ro(void)
> >   /* Adjustable memory block size */
> >   static unsigned long set_memory_block_size;
> > -int __init set_memory_block_size_order(unsigned int order)
> > +int set_memory_block_size_order(unsigned int order)
> >   {
> >   	unsigned long size = 1UL << order;
> > diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> > index 67858eeb92ed..f9045642f69e 100644
> > --- a/drivers/base/memory.c
> > +++ b/drivers/base/memory.c
> > @@ -110,6 +110,12 @@ static void memory_block_release(struct device *dev)
> >   	kfree(mem);
> >   }
> > +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.

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