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

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Message-ID: <d3203f2c-eff6-4e84-80cd-3c6f58dab292@redhat.com>
Date: Tue, 8 Oct 2024 17:02:33 +0200
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>
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 08.10.24 16:51, Gregory Price wrote:
> 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.

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). Just imagine having various users of such an interface ..

-- 
Cheers,

David / dhildenb