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

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Message-ID: <20200205231945.GB28446@richard>
Date:   Thu, 6 Feb 2020 07:19:45 +0800
From:   Wei Yang <richardw.yang@...ux.intel.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     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>,
        Baoquan He <bhe@...hat.com>,
        Wei Yang <richardw.yang@...ux.intel.com>
Subject: Re: [PATCH v1] mm/memory_hotplug: Easier calculation to get pages to
 next section boundary

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.

This function says the range should be block_size aligned. And if I am
correct, block size on x86 should be in the range

    [MIN_MEMORY_BLOCK_SIZE, MEM_SIZE_FOR_LARGE_BLOCK]
    
And MIN_MEMORY_BLOCK_SIZE is section size.

Seems currently we support subsection hotplug? Then how a subsection range got
hotplug? Or this patch is a pre-requisite?

>---
> mm/memory_hotplug.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>index 0a54ffac8c68..c30191183c04 100644
>--- a/mm/memory_hotplug.c
>+++ b/mm/memory_hotplug.c
>@@ -528,7 +528,8 @@ void __remove_pages(unsigned long pfn, unsigned long nr_pages,
> 	for (; pfn < end_pfn; pfn += cur_nr_pages) {
> 		cond_resched();
> 		/* Select all remaining pages up to the next section boundary */
>-		cur_nr_pages = min(end_pfn - pfn, -(pfn | PAGE_SECTION_MASK));
>+		cur_nr_pages = min(end_pfn - pfn,
>+				   SECTION_ALIGN_UP(pfn + 1) - pfn);
> 		__remove_section(pfn, cur_nr_pages, map_offset, altmap);
> 		map_offset = 0;
> 	}
>-- 
>2.24.1

-- 
Wei Yang
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