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

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Message-ID: <20200206022644.6u7pxf7by2w5trmi@master>
Date:   Thu, 6 Feb 2020 02:26:44 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Wei Yang <richardw.yang@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Easier calculation to get pages to
 next section boundary

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@...nel.crashing.org>
>> > >>Cc: Andrew Morton <akpm@...ux-foundation.org>
>> > >>Cc: Michal Hocko <mhocko@...nel.org>
>> > >>Cc: Oscar Salvador <osalvador@...e.de>
>> > >>Cc: Baoquan He <bhe@...hat.com>
>> > >>Cc: Wei Yang <richardw.yang@...ux.intel.com>
>> > >>Signed-off-by: David Hildenbrand <david@...hat.com>
>> > >
>> > >Reviewed-by: Wei Yang <richardw.yang@...ux.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 :-)

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.

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?

-- 
Wei Yang
Help you, Help me