vmcoreinfo.rst

==========
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 attributes. These flags are used to filter various unnecessary for
dumping pages.

PAGE_BUDDY_MAPCOUNT_VALUE(~PG_buddy)|PAGE_OFFLINE_MAPCOUNT_VALUE(~PG_offline)
-----------------------------------------------------------------------------

More 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.