Documentation/kdump/vmcoreinfo.rst - kernel/common - Git at Google

 ==========
 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.
	==========
	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.