Re: [PATCH v1] mm/memory_hotplug: Easier calculation to get pages to next section boundary

[Wei Yang]

On Thu, Feb 06, 2020 at 10:01:21AM +0100, David Hildenbrand wrote:
>On 06.02.20 05:39, Baoquan He wrote:
>> On 02/06/20 at 04:34am, Wei Yang wrote:
>>> On Thu, Feb 06, 2020 at 10:48:16AM +0800, Baoquan He wrote:
>>>> On 02/06/20 at 02:26am, Wei Yang wrote:
>>>>> On Thu, Feb 06, 2020 at 08:37:36AM +0800, Baoquan He wrote:
>>>>>> On 02/06/20 at 08:13am, Baoquan He wrote:
>>>>>>> On 02/06/20 at 07:50am, Wei Yang wrote:
>>>>>>>> On Thu, Feb 06, 2020 at 07:19:45AM +0800, Wei Yang wrote:
>>>>>>>>> On Wed, Feb 05, 2020 at 02:52:51PM +0100, David Hildenbrand wrote:
>>>>>>>>>> Let's use a calculation that's easier to understand and calculates the
>>>>>>>>>> same result. Reusing existing macros makes this look nicer.
>>>>>>>>>>
>>>>>>>>>> We always want to have the number of pages (> 0) to the next section
>>>>>>>>>> boundary, starting from the current pfn.
>>>>>>>>>>
>>>>>>>>>> Suggested-by: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>
>>>>>>>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>>>>>>>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>>>>>>>>>> Cc: Oscar Salvador <osalvador@xxxxxxx>
>>>>>>>>>> Cc: Baoquan He <bhe@xxxxxxxxxx>
>>>>>>>>>> Cc: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>>>>>>>>>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>>>>>>>>>
>>>>>>>>> Reviewed-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>>>>>>>>>
>>>>>>>>> BTW, I got one question about hotplug size requirement.
>>>>>>>>>
>>>>>>>>> I thought the hotplug range should be section size aligned, while taking a
>>>>>>>>> look into current code function check_hotplug_memory_range() guard the range.
>>>>>>>
>>>>>>> A good question. The current code should be block size aligned. I
>>>>>>> remember in some places we assume each block comprise all the sections.
>>>>>>> Can't imagine one or some of them are half section filled.
>>>>>>
>>>>>> I could be wrong, half filled block may not cause problem. 
>>>>>>
>>>>>
>>>>> David must be angry about our flooding the mail list :-)
>>>>
>>>> Believe he won't, :-) If you like, we can talk off line.
>>>>
>>>>>
>>>>> Check the code again, there are two memory range check:
>>>>>
>>>>>   * check_hotplug_memory_range(), block/section aligned
>>>>>   * check_pfn_span(), subsection aligned
>>>>>
>>>>> The second check, check_pfn_span() in __add_pages(), enable the capability to
>>>>> add a memory range with subsection size.
>>>>>
>>>>> This means hotplug still keeps section alignment.
>>>>
>>>> memremap_pages() also call add_pages(), it doesn't have the
>>>> check_hotplug_memory_range() invocation. check_pfn_span() is made for
>>>> it specifically.
>>>>
>>>
>>> If my understanding is correct, memremap_pages() is used to add some dev
>>> memory to system. This is the use case which Dan want to enable for
>>> sub-section. Since memremap_pages() is not called in mem-hotplug path, this
>>> doesn't affect the hotplug range alignment.
>> 
>> Yeah, we are on the same page.
>
>We allow sub-section hoy-add only for device memory/hmm. BIOS often
>align PMEM devices to sub-sections, and not supporting this makes life
>difficult to support these devices. (You cannot simply cut off something
>of a disk :) )
>
>System memory can only be hotplugged in memory block granularity, the
>same granularity onlining/offlining from user space will happen. Boot
>memory, however, can be sub-section aligned, but can never be offlined
>(as it contains holes) and therefore never be removed.
>

This makes life easier.

Thanks for your explanation.

-- 
Wei Yang
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