Re: [RFC PATCH] LoongArch: mm: avoid backing vmemmap beyond max_pfn

[Wentao Guan]

> > On Oct 21, 2026, at 14:21, Wentao Guan <guanwentao@xxxxxxxxxxxxx> wrote:
> >
> > A PMD maps 32MiB of vmemmap on LoongArch with 16KiB base pages, while a
> > 512MiB section only needs 2MiB.  vmemmap_populate() is called for each
> > section and vmemmap_populate_hugepages() installs a PMD mapping for the
> > whole PMD, so the last PMD also backs struct pages for PFNs beyond
> > max_pfn.
> >
> > On a 34GiB system 160MiB of vmemmap is mapped although 136MiB is
> > needed.  The extra 24MiB describes PFNs that do not exist.
> 
> Your commit messages are very detailed, but I don't really understand the
> problem you're trying to solve. What issue did you run into, and what was
> the resulting impact?
The commit message is misunderstanding, the issue is point to save memory cost,

> > On a 34GiB system 160MiB of vmemmap is mapped although 136MiB is
> > needed.  The extra 24MiB describes PFNs that do not exist.
On a 32GiB system the platform memory map leaves a 2GiB MMIO window
(serial 10080000,ACPI 100d0000,LOON000x... etc,)
below 4GiB and RAM ends at 34GiB, so max_pfn spans 34GiB.  The vmemmap
for that PFN range is 136MiB, but 160MiB is mapped; the extra 24MiB
describes PFNs that do not exist. In 8GB system, the cost is 0.37% (30MB/8GB).

BRs
Wentao Guan

Log:
 cat /proc/iomem 
00200000-0edfffff : System RAM
  00200000-01b6ffff : Kernel code
  01b70000-03c113ff : Kernel data
  03c11400-03d36caf : Kernel bss
  03d36cb0-03d3ffff : Reserved
  03d40000-03f4bfff : Reserved
10080000-100800ff : serial
10080100-100801ff : serial
10080200-100802ff : serial
10080300-100803ff : serial
10090000-10090007 : LOON0004:00
  10090000-10090007 : LOON0004:00 LOON0004:00
10090100-10090107 : LOON0004:01
  10090100-10090107 : LOON0004:01 LOON0004:01
10090200-10090207 : LOON0004:02
  10090200-10090207 : LOON0004:02 LOON0004:02
10090300-10090307 : LOON0004:03
  10090300-10090307 : LOON0004:03 LOON0004:03
10090400-10090407 : LOON0004:04
  10090400-10090407 : LOON0004:04 LOON0004:04
10090500-10090507 : LOON0004:05
  10090500-10090507 : LOON0004:05 LOON0004:05
100a0000-100a000f : LOON0006:00
  100a0000-100a000f : LOON0006:00 LOON0006:00
100a0100-100a010f : LOON0006:01
  100a0100-100a010f : LOON0006:01 LOON0006:01
100a0200-100a020f : LOON0006:02
  100a0200-100a020f : LOON0006:02 LOON0006:02
100a0300-100a030f : LOON0006:03
  100a0300-100a030f : LOON0006:03 LOON0006:03
100d000c-100d0013 : ACPI PM1a_EVT_BLK
100d0014-100d0017 : ACPI PM1a_CNT_BLK
100d0018-100d001b : ACPI PM_TMR
100d0028-100d002f : ACPI GPE0_BLK
100d0100-100d01ff : LOON0001:00
  100d0100-100d01ff : LOON0001:00 LOON0001:00
100e0000-100e0bff : LOON0002:00
  100e0000-100e0bff : LOON0002:00 LOON0002:00
1fe001e0-1fe001e7 : serial
90400000-f9b7ffff : System RAM
  ea4f0000-f7a9ffff : Reserved
  f9acc000-f9acffff : Reserved
  f9b00000-f9b7ffff : Reserved
f9b90000-fdabffff : System RAM
  f9b90000-f9bbffff : Reserved
fe080000-87fffffff : System RAM
  873800000-874023fff : Reserved
  874400000-874423fff : Reserved
  874800000-874823fff : Reserved
  874c00000-874c23fff : Reserved
  875000000-875023fff : Reserved
  875400000-875423fff : Reserved
  875800000-875823fff : Reserved
  875c00000-875c23fff : Reserved
  876000000-87dffffff : Reserved
  87e240000-87ff93fff : Reserved
  87ff94000-87ff97fff : Reserved
  87ff98000-87ff9bfff : Reserved
  87ff9c000-87ff9ffff : Reserved
  87ffa0000-87ffa3fff : Reserved
  87ffa4000-87ffa7fff : Reserved
  87ffa8000-87ffabfff : Reserved
  87ffac000-87ffaffff : Reserved
  87ffb0000-87ffb3fff : Reserved
  87ffb4000-87ffb7fff : Reserved
  87ffb8000-87ffbbfff : Reserved
  87ffbc000-87ffbffff : Reserved
  87ffc0000-87ffc3fff : Reserved
  87ffc4000-87ffc7fff : Reserved
  87ffc8000-87ffcbfff : Reserved
  87ffcc000-87ffcffff : Reserved
  87ffd0000-87ffd3fff : Reserved
  87ffd4000-87ffd7fff : Reserved
  87ffd8000-87ffdbfff : Reserved
  87ffdc000-87ffdffff : Reserved
  87ffe0000-87ffe3fff : Reserved
  87ffe4000-87ffe7fff : Reserved
  87ffe8000-87ffebfff : Reserved
  87ffec000-87ffeffff : Reserved
  87fff0000-87fff3fff : Reserved
  87fff4000-87fff7fff : Reserved
  87fff8000-87fffbfff : Reserved
  87fffc000-87fffffff : Reserved
e0030000000-e007fffffff : PCI Bus 0000:00
  e0030000000-e00300fffff : PCI Bus 0000:02
    e0030000000-e0030003fff : 0000:02:00.0
    e0030004000-e0030004fff : 0000:02:00.0
      e0030004000-e0030004fff : r8169
  e0030100000-e00301007ff : 0000:00:1a.0
  e0030100800-e0030100bff : 0000:00:08.0
    e0030100800-e0030100bff : ahci
e8000000000-efcffffffff : PCI Bus 0000:00
  e8000000000-e800fffffff : 0000:00:06.0
    e8000000000-e80007ebfff : BOOTFB
      e8000000000-e80007ebfff : simpledrm
  e8010000000-e8013ffffff : 0000:00:1a.0
  e8014000000-e8014ffffff : 0000:00:16.0
  e8015000000-e80150fffff : 0000:00:19.0
    e8015000000-e80150fffff : xhci-hcd
  e8015100000-e801510ffff : 0000:00:06.0
  e8015110000-e801511ffff : 0000:00:06.1
  e8015120000-e801512ffff : 0000:00:06.2
    e8015120000-e801512ffff : ICH HD audio
  e8015130000-e801513ffff : 0000:00:07.0
    e8015130000-e801513ffff : ICH HD audio
  e8015140000-e8015147fff : 0000:00:03.0
    e8015140000-e8015147fff : dwmac-loongson-pci
  e8015148000-e801514ffff : 0000:00:04.0
    e8015148000-e801514ffff : ohci_hcd
  e8015150000-e8015157fff : 0000:00:04.1
    e8015150000-e8015157fff : ehci_hcd
  e8015158000-e801515ffff : 0000:00:05.0
    e8015158000-e801515ffff : ohci_hcd
  e8015160000-e8015167fff : 0000:00:05.1
    e8015160000-e8015167fff : ehci_hcd
  e8015168000-e8015168fff : 0000:00:09.0
  e8015169000-e8015169fff : 0000:00:0a.0
  e801516a000-e801516afff : 0000:00:0b.0
  e801516b000-e801516bfff : 0000:00:0c.0
  e801516c000-e801516cfff : 0000:00:0d.0
  e801516d000-e801516dfff : 0000:00:0f.0
  e801516e000-e801516efff : 0000:00:10.0
  e801516f000-e801516ffff : 0000:00:13.0
  e8015170000-e8015170fff : 0000:00:16.0
    e8015170000-e8015170fff : 0000:00:16.0
  e8015171000-e80151713ff : 0000:00:1a.0
  e8015171400-e80151714ff : 0000:00:06.0
efe00000000-efe1fffffff : PCI ECAM