Re: [PATCH v3] mm/memory hotplug/unplug: Optimize zone contiguous check when changing pfn range

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Apr 13, 2026 at 08:24:05PM +0200, David Hildenbrand (Arm) wrote:
>>   With the last memblock region fits in Node 1 Zone Normal. 
>> 
>>   Then I punch a hole in this region with 2M(subsection) size with following
>>   change, to mimic there is a hole in memory range:
>> 
>>     @@ -1372,5 +1372,8 @@ __init void e820__memblock_setup(void)
>>             /* Throw away partial pages: */
>>             memblock_trim_memory(PAGE_SIZE);
>>      
>>     +       memblock_remove(0x140000000, 0x200000);
>>     +
>>             memblock_dump_all();
>>      }
>>     
>>   Then the memblock dump shows:
>> 
>>      MEMBLOCK configuration:
>>       memory size = 0x000000017fd7dc00 reserved size = 0x0000000005a97 9c2
>>       memory.cnt  = 0x4
>>       memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
>>       memory[0x1]     [0x0000000000100000-0x00000000bffdefff], 0x00000000bfedf000 bytes on node 0 flags: 0x0
>>   +-  memory[0x2]     [0x0000000100000000-0x000000013fffffff], 0x0000000040000000 bytes on node 1 flags: 0x0
>>   +-  memory[0x3]     [0x0000000140200000-0x00000001bfffffff], 0x000000007fe00000 bytes on node 1 flags: 0x0
>> 
>>   We can see the original one memblock region is divided into two, with a hole
>>   of 2M in the middle.
>
>Yes, that makes sense.
>
>> 
>>   Not sure this is a reasonable mimic of memory hole. Also I tried to
>>   punch a larger hole, e.g. 10M, still see the behavioral change.
>> 
>> The /proc/zoneinfo result:
>> 
>>     w/o patch
>> 
>>     Node 1, zone   Normal
>>       pages free     469271
>>             boost    0
>>             min      8567
>>             low      10708
>>             high     12849
>>             promo    14990
>>             spanned  786432
>>             present  785920
>>             contigu  0           <--- zone is non-contiguous
>>             managed  766024
>>             cma      0
>>     
>>     with patch
>> 
>>     Node 1, zone   Normal
>>       pages free     121098
>>             boost    0
>>             min      8665
>>             low      10831
>>             high     12997
>>             promo    15163
>>             spanned  786432
>>             present  785920
>>             contigu  1           <--- zone is contiguous
>>             managed  773041
>>             cma      0
>> 
>>   This shows we treat Node 1 Zone Normal as non-contiguous before, but treat
>>   it a contiguous zone after this patch.
>> 
>> Reason:
>> 
>>   set_zone_contiguous()
>>       __pageblock_pfn_to_page()
>>           pfn_to_online_page()
>> 	      pfn_section_valid() <--- check subsection
>> 
>>   When SPARSEMEM_VMEMMEP is set, pfn_section_valid() checks subsection bit to
>>   decide if it is valid. For a hole, the corresponding bit is not set. So it
>>   is non-contiguous before the patch.
>> 
>>   After this patch, the memory map in this hole also contributes to
>>   pages_with_online_memmap, so it is treated as contiguous.
>
>That means that mm init code actually initialized a memmap, so there is
>a memmap there that is properly initialized?
>
>So init_unavailable_range()->for_each_valid_pfn() processed these
>sub-section holes I guess.
>

Yes, I think so.

When memmap_init()->for_each_mem_pfn_range() iterate on the last memblock
region, init_unavailable_range() will init the hole.

>subsection_map_init() takes care of initializing the subsections. That
>happens before memmap_init() in free_area_init().
>

Yes. I guess you mean sparse_init_subsection_map().

>Is there a problem in for_each_valid_pfn()?
>
>And I think there is in first_valid_pfn:
>

You mean there is a problem in first_valid_pfn?

>	if (valid_section(ms) &&
>	    (early_section(ms) || pfn_section_first_valid(ms, &pfn))) {
>		rcu_read_unlock_sched();
>		return pfn;
>	}
>
>The PFN is valid, but we actually care about whether it will be online.
>So likely, we should skip over sub-sections here also for early sections
>(even though the memmap exist, nobody should be looking at it, just like
>for an offline memory section).
>

And it should be like below?

	if (valid_section(ms) &&
	    pfn_section_first_valid(ms, &pfn)) {
		rcu_read_unlock_sched();
		return pfn;
	}

IIUC, this would skip hole and leave allocated memory map uninitialized. And
then those pages won't contribute to pages_with_online_memmap, which further
leave the zone non-contiguous.

But we want zone to be contiguous when we have a hole like this, right?

Sorry, I don't follow here.

>> 
>> Some question:
>> 
>>   I suspect with !SPARSEMEM_VMEMMEP, we always treat Zone Normal as
>>   contiguous, because we don't set subsection. So it looks the behavior is
>>   different from SPARSEMEM_VMEMMEP. But I didn't manage to build kernel with
>>   !SPARSEMEM_VMEMMEP to verify.
>> 
>>   I see the discussion on defining zone->contiguous as safe to use
>>   pfn_to_page() for the whole zone. For this purpose, current change looks
>>   good to me. Since we do allocate and init memory map for holes.
>
>Right.
>
>> 
>>   But pageblock_pfn_to_page() is used for compaction and other. A pfn with
>>   memory map but no actual memory seems not guarantee to be a usable page. So
>>   the correct usage of pageblock_pfn_to_page() is after
>>   pageblock_pfn_to_page() return a page, we should validate each page in the
>>   range before using? I am a little lost here.
>
>These non-existent pages (holes) are no different than allocated
>un-movable memory. So compaction code must deal with them. Just like
>smaller memory holes that don't cover a full memory section.
>

Thanks for explanation.

>-- 
>Cheers,
>
>David

-- 
Wei Yang
Help you, Help me