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Date:   Thu, 6 Feb 2020 04:34:01 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Baoquan He <bhe@redhat.com>
Cc:     Wei Yang <richard.weiyang@gmail.com>,
        Wei Yang <richardw.yang@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Oscar Salvador <osalvador@suse.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Easier calculation to get pages to
 next section boundary
Message-ID: <20200206043401.22i2cucwqctsrtps@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
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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@kernel.crashing.org>
>> >> > >>Cc: Andrew Morton <akpm@linux-foundation.org>
>> >> > >>Cc: Michal Hocko <mhocko@kernel.org>
>> >> > >>Cc: Oscar Salvador <osalvador@suse.de>
>> >> > >>Cc: Baoquan He <bhe@redhat.com>
>> >> > >>Cc: Wei Yang <richardw.yang@linux.intel.com>
>> >> > >>Signed-off-by: David Hildenbrand <david@redhat.com>
>> >> > >
>> >> > >Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>
>> >> > >
>> >> > >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.

>> 
>> BTW, __add_pages() share the same logic as __remove_pages(). Why not change it
>> too? Do I miss something or I don't have the latest source code?
>
>Good question, and I think it need. Just David is refactoring/cleaning
>up the remove_pages() code path, this is found out by Segher from patch
>reviewing.

Ah, we may need a following cleanup :-)

-- 
Wei Yang
Help you, Help me