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

[Wei Yang]

On Tue, Apr 07, 2026 at 11:16:15PM -0400, Yuan Liu wrote:
[...]
> 
>-void set_zone_contiguous(struct zone *zone)
>-{
>-	unsigned long block_start_pfn = zone->zone_start_pfn;
>-	unsigned long block_end_pfn;
>-
>-	block_end_pfn = pageblock_end_pfn(block_start_pfn);
>-	for (; block_start_pfn < zone_end_pfn(zone);
>-			block_start_pfn = block_end_pfn,
>-			 block_end_pfn += pageblock_nr_pages) {
>-
>-		block_end_pfn = min(block_end_pfn, zone_end_pfn(zone));
>-
>-		if (!__pageblock_pfn_to_page(block_start_pfn,
>-					     block_end_pfn, zone))
>-			return;
>-		cond_resched();
>-	}
>-
>-	/* We confirm that there is no hole */
>-	zone->contiguous = true;
>-}
>-

Hi, 

I may see a behavioral change after this patch.

  * An originally non-contiguous zone would be detected as contiguous after this patch.

My test setup:

  Did test in a qemu with 6G memory with memblock_debug enabled.
  And adjust the /proc/zoneinfo to display zone->contiguous field.

  Originally, memblock_dump shows:

     MEMBLOCK configuration:
      memory size = 0x000000017ff7dc00 reserved size = 0x0000000005a9d9c2
      memory.cnt  = 0x3
      memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
      memory[0x1]     [0x0000000000100000-0x00000000bffdefff], 0x00000000bfedf000 bytes on node 0 flags: 0x0
  +-  memory[0x2]     [0x0000000100000000-0x00000001bfffffff], 0x00000000c0000000 bytes on node 1 flags: 0x0
    
  And zone range shows:

     Zone ranges:
       DMA      [mem 0x0000000000001000-0x0000000000ffffff]
       DMA32    [mem 0x0000000001000000-0x00000000ffffffff]
       Normal   [mem 0x0000000100000000-0x00000001bfffffff]     <--- entire last memblock region

  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.

  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.

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.

  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.


> /*
>  * Check if a PFN range intersects multiple zones on one or more
>  * NUMA nodes. Specify the @nid argument if it is known that this
>-- 
>2.47.3

-- 
Wei Yang
Help you, Help me