kdump: Document kernel data exported in the vmcoreinfo note

================================================================

			VMCOREINFO

================================================================

===========

What is it?

===========

VMCOREINFO is a special ELF note section. It contains various

information from the kernel like structure size, page size, symbol

values, field offsets, etc. These data are packed into an ELF note

section and used by user-space tools like crash and makedumpfile to

analyze a kernel's memory layout.

================

Common variables

================

init_uts_ns.name.release

------------------------

The version of the Linux kernel. Used to find the corresponding source

code from which the kernel has been built. For example, crash uses it to

find the corresponding vmlinux in order to process vmcore.

PAGE_SIZE

---------

The size of a page. It is the smallest unit of data used by the memory

management facilities. It is usually 4096 bytes of size and a page is

aligned on 4096 bytes. Used for computing page addresses.

init_uts_ns

-----------

The UTS namespace which is used to isolate two specific elements of the

system that relate to the uname(2) system call. It is named after the

data structure used to store information returned by the uname(2) system

call.

User-space tools can get the kernel name, host name, kernel release

number, kernel version, architecture name and OS type from it.

node_online_map

---------------

An array node_states[N_ONLINE] which represents the set of online nodes

in a system, one bit position per node number. Used to keep track of

which nodes are in the system and online.

swapper_pg_dir

-------------

The global page directory pointer of the kernel. Used to translate

virtual to physical addresses.

_stext

------

Defines the beginning of the text section. In general, _stext indicates

the kernel start address. Used to convert a virtual address from the

direct kernel map to a physical address.

vmap_area_list

--------------

Stores the virtual area list. makedumpfile gets the vmalloc start value

from this variable and its value is necessary for vmalloc translation.

mem_map

-------

Physical addresses are translated to struct pages by treating them as

an index into the mem_map array. Right-shifting a physical address

PAGE_SHIFT bits converts it into a page frame number which is an index

into that mem_map array.

Used to map an address to the corresponding struct page.

contig_page_data

----------------

Makedumpfile gets the pglist_data structure from this symbol, which is

used to describe the memory layout.

User-space tools use this to exclude free pages when dumping memory.

mem_section|(mem_section, NR_SECTION_ROOTS)|(mem_section, section_mem_map)

--------------------------------------------------------------------------

The address of the mem_section array, its length, structure size, and

the section_mem_map offset.

It exists in the sparse memory mapping model, and it is also somewhat

similar to the mem_map variable, both of them are used to translate an

address.

page

----

The size of a page structure. struct page is an important data structure

and it is widely used to compute contiguous memory.

pglist_data

-----------

The size of a pglist_data structure. This value is used to check if the

pglist_data structure is valid. It is also used for checking the memory

type.

zone

----

The size of a zone structure. This value is used to check if the zone

structure has been found. It is also used for excluding free pages.

free_area

---------

The size of a free_area structure. It indicates whether the free_area

structure is valid or not. Useful when excluding free pages.

list_head

---------

The size of a list_head structure. Used when iterating lists in a

post-mortem analysis session.

nodemask_t

----------

The size of a nodemask_t type. Used to compute the number of online

nodes.

(page, flags|_refcount|mapping|lru|_mapcount|private|compound_dtor|

       compound_order|compound_head)

-------------------------------------------------------------------

User-space tools compute their values based on the offset of these

variables. The variables are used when excluding unnecessary pages.

(pglist_data, node_zones|nr_zones|node_mem_map|node_start_pfn|node_

              spanned_pages|node_id)

-------------------------------------------------------------------

On NUMA machines, each NUMA node has a pg_data_t to describe its memory

layout. On UMA machines there is a single pglist_data which describes the

whole memory.

These values are used to check the memory type and to compute the

virtual address for memory map.

(zone, free_area|vm_stat|spanned_pages)

---------------------------------------

Each node is divided into a number of blocks called zones which

represent ranges within memory. A zone is described by a structure zone.

User-space tools compute required values based on the offset of these

variables.

(free_area, free_list)

----------------------

Offset of the free_list's member. This value is used to compute the number

of free pages.

Each zone has a free_area structure array called free_area[MAX_ORDER].

The free_list represents a linked list of free page blocks.

(list_head, next|prev)

----------------------

Offsets of the list_head's members. list_head is used to define a

circular linked list. User-space tools need these in order to traverse

lists.

(vmap_area, va_start|list)

--------------------------

Offsets of the vmap_area's members. They carry vmalloc-specific

information. Makedumpfile gets the start address of the vmalloc region

from this.

(zone.free_area, MAX_ORDER)

---------------------------

Free areas descriptor. User-space tools use this value to iterate the

free_area ranges. MAX_ORDER is used by the zone buddy allocator.

log_first_idx

-------------

Index of the first record stored in the buffer log_buf. Used by

user-space tools to read the strings in the log_buf.

log_buf

-------

Console output is written to the ring buffer log_buf at index

log_first_idx. Used to get the kernel log.

log_buf_len

-----------

log_buf's length.

clear_idx

---------

The index that the next printk() record to read after the last clear

command. It indicates the first record after the last SYSLOG_ACTION

_CLEAR, like issued by 'dmesg -c'. Used by user-space tools to dump

the dmesg log.

log_next_idx

------------

The index of the next record to store in the buffer log_buf. Used to

compute the index of the current buffer position.

printk_log

----------

The size of a structure printk_log. Used to compute the size of

messages, and extract dmesg log. It encapsulates header information for

log_buf, such as timestamp, syslog level, etc.

(printk_log, ts_nsec|len|text_len|dict_len)

-------------------------------------------

It represents field offsets in struct printk_log. User space tools

parse it and check whether the values of printk_log's members have been

changed.

(free_area.free_list, MIGRATE_TYPES)

------------------------------------

The number of migrate types for pages. The free_list is described by the

array. Used by tools to compute the number of free pages.

NR_FREE_PAGES

-------------

On linux-2.6.21 or later, the number of free pages is in

vm_stat[NR_FREE_PAGES]. Used to get the number of free pages.

PG_lru|PG_private|PG_swapcache|PG_swapbacked|PG_slab|PG_hwpoision

|PG_head_mask|PAGE_BUDDY_MAPCOUNT_VALUE(~PG_buddy)

|PAGE_OFFLINE_MAPCOUNT_VALUE(~PG_offline)

-----------------------------------------------------------------

Page attributes. These flags are used to filter various unnecessary for

dumping pages.

HUGETLB_PAGE_DTOR

-----------------

The HUGETLB_PAGE_DTOR flag denotes hugetlbfs pages. Makedumpfile

excludes these pages.

======

x86_64

======

phys_base

---------

Used to convert the virtual address of an exported kernel symbol to its

corresponding physical address.

init_top_pgt

------------

Used to walk through the whole page table and convert virtual addresses

to physical addresses. The init_top_pgt is somewhat similar to

swapper_pg_dir, but it is only used in x86_64.

pgtable_l5_enabled

------------------

User-space tools need to know whether the crash kernel was in 5-level

paging mode.

node_data

---------

This is a struct pglist_data array and stores all NUMA nodes

information. Makedumpfile gets the pglist_data structure from it.

(node_data, MAX_NUMNODES)

-------------------------

The maximum number of nodes in system.

KERNELOFFSET

------------

The kernel randomization offset. Used to compute the page offset. If

KASLR is disabled, this value is zero.

KERNEL_IMAGE_SIZE

-----------------

Currently unused by Makedumpfile. Used to compute the module virtual

address by Crash.

sme_mask

--------

AMD-specific with SME support: it indicates the secure memory encryption

mask. Makedumpfile tools need to know whether the crash kernel was

encrypted. If SME is enabled in the first kernel, the crash kernel's

page table entries (pgd/pud/pmd/pte) contain the memory encryption

mask. This is used to remove the SME mask and obtain the true physical

address.

Currently, sme_mask stores the value of the C-bit position. If needed,

additional SME-relevant info can be placed in that variable.

For example:

[ misc	        ][ enc bit  ][ other misc SME info       ]

0000_0000_0000_0000_1000_0000_0000_0000_0000_0000_..._0000

63   59   55   51   47   43   39   35   31   27   ... 3

======

x86_32

======

X86_PAE

-------

Denotes whether physical address extensions are enabled. It has the cost

of a higher page table lookup overhead, and also consumes more page

table space per process. Used to check whether PAE was enabled in the

crash kernel when converting virtual addresses to physical addresses.

====

ia64

====

pgdat_list|(pgdat_list, MAX_NUMNODES)

-------------------------------------

pg_data_t array storing all NUMA nodes information. MAX_NUMNODES

indicates the number of the nodes.

node_memblk|(node_memblk, NR_NODE_MEMBLKS)

------------------------------------------

List of node memory chunks. Filled when parsing the SRAT table to obtain

information about memory nodes. NR_NODE_MEMBLKS indicates the number of

node memory chunks.

These values are used to compute the number of nodes the crashed kernel used.

node_memblk_s|(node_memblk_s, start_paddr)|(node_memblk_s, size)

----------------------------------------------------------------

The size of a struct node_memblk_s and the offsets of the

node_memblk_s's members. Used to compute the number of nodes.

PGTABLE_3|PGTABLE_4

-------------------

User-space tools need to know whether the crash kernel was in 3-level or

4-level paging mode. Used to distinguish the page table.

=====

ARM64

=====

VA_BITS

-------

The maximum number of bits for virtual addresses. Used to compute the

virtual memory ranges.

kimage_voffset

--------------

The offset between the kernel virtual and physical mappings. Used to

translate virtual to physical addresses.

PHYS_OFFSET

-----------

Indicates the physical address of the start of memory. Similar to

kimage_voffset, which is used to translate virtual to physical

addresses.

KERNELOFFSET

------------

The kernel randomization offset. Used to compute the page offset. If

KASLR is disabled, this value is zero.

====

arm

====

ARM_LPAE

--------

It indicates whether the crash kernel supports large physical address

extensions. Used to translate virtual to physical addresses.

====

s390

====

lowcore_ptr

----------

An array with a pointer to the lowcore of every CPU. Used to print the

psw and all registers information.

high_memory

-----------

Used to get the vmalloc_start address from the high_memory symbol.

(lowcore_ptr, NR_CPUS)

----------------------

The maximum number of CPUs.

=======

powerpc

=======

node_data|(node_data, MAX_NUMNODES)

-----------------------------------

See above.

contig_page_data

----------------

See above.

vmemmap_list

------------

The vmemmap_list maintains the entire vmemmap physical mapping. Used

to get vmemmap list count and populated vmemmap regions info. If the

vmemmap address translation information is stored in the crash kernel,

it is used to translate vmemmap kernel virtual addresses.

mmu_vmemmap_psize

-----------------

The size of a page. Used to translate virtual to physical addresses.

mmu_psize_defs

--------------

Page size definitions, i.e. 4k, 64k, or 16M.

Used to make vtop translations.

vmemmap_backing|(vmemmap_backing, list)|(vmemmap_backing, phys)|

(vmemmap_backing, virt_addr)

----------------------------------------------------------------

The vmemmap virtual address space management does not have a traditional

page table to track which virtual struct pages are backed by a physical

mapping. The virtual to physical mappings are tracked in a simple linked

list format.

User-space tools need to know the offset of list, phys and virt_addr

when computing the count of vmemmap regions.

mmu_psize_def|(mmu_psize_def, shift)

------------------------------------

The size of a struct mmu_psize_def and the offset of mmu_psize_def's

member.

Used in vtop translations.

==

sh

==

node_data|(node_data, MAX_NUMNODES)

-----------------------------------

See above.

X2TLB

-----

Indicates whether the crashed kernel enabled SH extended mode.