RE: [PATCH 1/2 v5] kdump: add the vmcoreinfo documentation

[Hatayama, Daisuke]

Hi,

> -----Original Message-----
> From: linux-kernel-owner@xxxxxxxxxxxxxxx
> [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of Lianbo Jiang
> Sent: Monday, January 7, 2019 10:48 AM
> To: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kexec@xxxxxxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx;
> bp@xxxxxxxxx; x86@xxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; bhe@xxxxxxxxxx;
> dyoung@xxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx; k-hagio@xxxxxxxxxxxxx;
> anderson@xxxxxxxxxx
> Subject: [PATCH 1/2 v5] kdump: add the vmcoreinfo documentation
> 
> This document lists some variables that export to vmcoreinfo, and briefly
> describles what these variables indicate. It should be instructive for
> many people who do not know the vmcoreinfo, and it also normalizes the

I agree to this part, but

> exported variables as a convention between kernel and use-space.

I don't agree to this part.

The meaning of each symbol is decided by each feature in the kernel,
not by vmcoreinfo. I suspect anyone mistakenly understand this document is
ABI enforcing each symbol works as described. We can change symbols and
their meaning regardless of this document.

Oh, I found this topic has already been discussed at v3, and
you removed "ABI" in the patch description at v4.

But it seems still confusing to me.
I think the explicit description saying that this is for user-land tools,
they treats each symbol as described,
and the document never affect implementation of each kernel components,
is necessary in e.g. "Purpose of this document" section?

> 
> Suggested-by: Borislav Petkov <bp@xxxxxxx>
> Signed-off-by: Lianbo Jiang <lijiang@xxxxxxxxxx>
> ---
>  Documentation/kdump/vmcoreinfo.txt | 500 +++++++++++++++++++++++++++++
>  1 file changed, 500 insertions(+)
>  create mode 100644 Documentation/kdump/vmcoreinfo.txt
> 
> diff --git a/Documentation/kdump/vmcoreinfo.txt
> b/Documentation/kdump/vmcoreinfo.txt
> new file mode 100644
> index 000000000000..8e444586b87b
> --- /dev/null
> +++ b/Documentation/kdump/vmcoreinfo.txt
> @@ -0,0 +1,500 @@
> +================================================================
> +			VMCOREINFO
> +================================================================
> +
> +=======================
> +What is the VMCOREINFO?
> +=======================
> +
> +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.
> +
> +PAGE_SIZE
> +---------
> +
> +The size of a page. It is the smallest unit of data for memory
> +management in kernel. It is usually 4096 bytes and a page is aligned
> +on 4096 bytes. Used for computing page addresses.
> +
> +init_uts_ns
> +-----------
> +
> +This is the UTS namespace, which is used to isolate two specific
> +elements of the system that relate to the uname(2) system call. The UTS
> +namespace 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 node
> +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 can get the vmalloc start
> +value from this variable. This 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 can get 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 the contiguous memory.
> +
> +pglist_data
> +-----------
> +
> +The size of a pglist_data structure. This value will be 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 often 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 for 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 can compute their values based on the offset of these
> +variables. The variables are helpful to exclude 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 they are also helpful
> +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.
> +Each zone type is suitable for a different type of usage.
> +
> +User-space tools can 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 fields in this structure are simple, 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 indicate the vmalloc layer
> +information. Makedumpfile gets the start address of the vmalloc region.
> +
> +(zone.free_area, MAX_ORDER)
> +---------------------------
> +
> +It indicates the maximum number of the array free_area. This macro is
> +used by the zone buddy allocator. User-space tools use this value to
> +iterate the free_area.
> +
> +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
> +-----------
> +
> +Length of a log_buf. Used to read the number of strings from the
> +log_buf.
> +
> +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.
> +
> +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 string position.
> +
> +printk_log
> +----------
> +
> +The size of a structure printk_log. Used to compute the size of
> +messages, and extract dmesg log. It can output human readable text.
> +Encapsulate 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 can
> +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 divided into
> +the array, it needs to know the number of the array when makedumpfile
> +computes 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
> +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
> +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 the
> +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 the 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
> +--------
> +
> +For AMD machine with SME feature, 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 (pgd/pud/pmd/pte) contains the memory encryption
> +mask and this is used to remove the SME mask to obtain the true physical
> +address.
> +
> +Currently, the sme_mask stores the value of sme_me_mask(bit 47). If need,
> +the bit(sme_mask) might be redefined in the future, but the bit 63 will
> +be reserved.
> +
> +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 more 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 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 in the crash kernel.
> +
> +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 address to physical
> +address.
> +
> +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 address to physical address.
> +
> +====
> +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. It can
> +get vmemmap list count and populate vmemmap regions info. If the vmemmap
> +address translation information is stored in the crash kernel, it helps
> +to translate vmemmap kernel virtual addresses.
> +
> +mmu_vmemmap_psize
> +-----------------
> +
> +The size of a page. Used to translate address 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 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 crash kernel enables SH extended mode.
> --
> 2.17.1
>