Re: [PATCH v2 2/3] x86: probe memblock size advisement value during mm init

[David Hildenbrand <david@redhat.com>]

On 21.10.24 16:46, Gregory Price wrote:
> On Mon, Oct 21, 2024 at 01:12:26PM +0200, David Hildenbrand wrote:
>>
>>
>> Am 16.10.24 um 21:24 schrieb Gregory Price:
>>> Systems with hotplug may provide an advisement value on what the
>>> memblock size should be.  Probe this value when the rest of the
>>> configuration values are considered.
>>>
>>> The new heuristic is as follows
>>>
>>> 1) set_memory_block_size_order value if already set (cmdline param)
>>> 2) minimum block size if memory is less than large block limit
>>> 3) [new] hotplug advise: lesser of advise value or memory alignment
>>> 4) Max block size if system is bare-metal
>>> 5) Largest size that aligns to end of memory.
>>>
>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>> Signed-off-by: Gregory Price <gourry@gourry.net>
>>> ---
>>>    arch/x86/mm/init_64.c | 16 ++++++++++++++++
>>>    1 file changed, 16 insertions(+)
>>>
>>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
>>> index ff253648706f..b72923b12d99 100644
>>> --- a/arch/x86/mm/init_64.c
>>> +++ b/arch/x86/mm/init_64.c
>>> @@ -1439,6 +1439,7 @@ static unsigned long probe_memory_block_size(void)
>>>    {
>>>    	unsigned long boot_mem_end = max_pfn << PAGE_SHIFT;
>>>    	unsigned long bz;
>>> +	int order;
>>>    	/* If memory block size has been set, then use it */
>>>    	bz = set_memory_block_size;
>>> @@ -1451,6 +1452,21 @@ static unsigned long probe_memory_block_size(void)
>>>    		goto done;
>>>    	}
>>> +	/* Consider hotplug advisement value (if set) */
>>> +	order = memblock_probe_size_order();
>>
>> "size_order" is a very weird name. Just return a size?
>>
>> memory_block_advised_max_size()
>>
>> or sth like that?
>>
> 
> There isn't technically an overall "max block size", nor any alignment
> requirements - so order was a nice way of enforcing 2-order alignment
> while also having the ability to get a -1/-EBUSY/whatever out.

I see. But we (MM) just call it "order" then, like pageblock_order, 
max_order, compound_order ... but here we use "size everywhere" so I 
prefer to just sticking to that.

> 
> I can change it if it's a big sticking point - but that's my reasoning.

Simply enforce it when setting the size. We call it "memory_block_size" 
everywhere and it's also a power-of-2 etc and sanity-check that in 
memory_dev_init().


-- 
Cheers,

David / dhildenb