Re: [PATCH 2 of 4] hotplug-memory: adding non-section-aligned memory is bad

[Jeremy Fitzhardinge <jeremy@goop.org>]

KAMEZAWA Hiroyuki wrote:
> On Fri, 28 Mar 2008 10:34:51 +0900
> Yasunori Goto <y-goto@jp.fujitsu.com> wrote:
>   
>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>>> --- a/mm/memory_hotplug.c
>>>> +++ b/mm/memory_hotplug.c
>>>> @@ -300,6 +300,11 @@
>>>>  	int ret;
>>>>  	u64 start = res->start;
>>>>  	u64 size = res->end - res->start + 1;
>>>> +
>>>> +	/* Adding non-section-aligned memory will give unexpected
>>>> +	   and unintuitive results. */
>>>> +	WARN_ON((start & SECTION_SIZE_MASK) != 0);
>>>> +	WARN_ON((size & SECTION_SIZE_MASK) != 0);
>>>>  
>>>>         
>>> Why just WARNING ? not BUG_ON?
>>>       
>> Both Nack.
>>
>> Because, firmware may occupy some area in the section.
>> Firmware must exclude those area to notify kernel. So, E820, EFI,
>> or _CRS of ACPI may return not aligned address and size.
>> register_memory_resource() and walk_memory_resource() are to skip
>> them silently. This is intended.
>>
>>     
> Ah, ok. sorry.
>
> Jeremy, I think you can check whether you have 'struct page' or not by
> pfn_valid(). 
>
> If pfn_valid() == false, you should call add_memory() and create
> a section/mem_map. If pfn_valid() == true, you should just remove
> PG_reserved bit in mem_map by online_page().

OK.  Would that ever be necessary if I explicitly align my start and size?

    J