* The kdump-compressed format
  makedumpfile provides two DUMPFILE formats (the ELF format and the
  kdump-compressed format). By default, makedumpfile makes a DUMPFILE
  in the kdump-compressed format. The kdump-compressed format is readable
  only with the crash utility, and it can be smaller than the ELF format
  because of the compression support.

  - The file structure
                                                 File offset
    +------------------------------------------+ 0x0
    |    main header (struct disk_dump_header) |
    |------------------------------------------+ block 1
    |    sub header (struct kdump_sub_header)  |
    |------------------------------------------+ block 2
    |            1st-bitmap                    |
    |------------------------------------------+ block 2 + X blocks
    |            2nd-bitmap                    | (aligned by block)
    |------------------------------------------+ block 2 + 2 * X blocks
    | page header for pfn 0 (struct page_desc) | (aligned by block)
    | page header for pfn 1 (struct page_desc) |
    |                     :                    |
    | page header for pfn Z (struct page_desc) |
    |------------------------------------------| (not aligned by block)
    |         page data (pfn 0)                |
    |         page data (pfn 1)                |
    |                        :                 |
    |         page data (pfn Z)                |
    +------------------------------------------+ offset_eraseinfo
    | erase mystruct2.mystruct1.var size 4     |
    | erase mystruct2.mystruct1.ptr nullify    |
    | erase mystruct2.mystruct.array size 100  |
    +------------------------------------------+


  - main header
    The main header of the kdump compressed format is the almost same as the
    one of diskdump. This header has the following members, and the member
    signature and header_version are different from diskdump.

    struct disk_dump_header {
        char                    signature[SIG_LEN];     /* = "KDUMP   " */
        int                     header_version; /* Dump header version */
        struct new_utsname      utsname;        /* copy of system_utsname */
        struct timeval          timestamp;      /* Time stamp */
        unsigned int            status;         /* Above flags */
        int                     block_size;     /* Size of a block in byte */
        int                     sub_hdr_size;   /* Size of arch dependent
                                                   header in blocks */
        unsigned int            bitmap_blocks;  /* Size of Memory bitmap in
                                                   block */
        unsigned int            max_mapnr;      /* = max_mapnr */
        unsigned int            total_ram_blocks;/* Number of blocks should be
                                                   written */
        unsigned int            device_blocks;  /* Number of total blocks in
                                                 * the dump device */
        unsigned int            written_blocks; /* Number of written blocks */
        unsigned int            current_cpu;    /* CPU# which handles dump */
        int                     nr_cpus;        /* Number of CPUs */
        struct task_struct      *tasks[0];
    };

  - sub header
    The sub header of the kdump compressed format is original. This header
    has the member phys_base and dump_level. The member phys_base is for
    an x86_64 relocatable kernel, and the member dump_level has '-d' option's
    value of makedumpfile command.

    struct kdump_sub_header {
        unsigned long   phys_base;
        int             dump_level;     /* header_version 1 and later */
	int		split;		/* header_version 2 and later */
	unsigned long	start_pfn;	/* header_version 2 and later */
	unsigned long	end_pfn;	/* header_version 2 and later */
	off_t		offset_vmcoreinfo;/* header_version 3 and later */
	unsigned long	size_vmcoreinfo;  /* header_version 3 and later */
	off_t		offset_note;      /* header_version 4 and later */
	unsigned long	size_note;        /* header_version 4 and later */
	off_t		offset_eraseinfo; /* header_version 5 and later */
	unsigned long	size_eraseinfo;   /* header_version 5 and later */
    };

  - 1st-bitmap
    The bit of 1st-bitmap presents either a page on memory hole, or not.
    If a page is on memory hole, the corresponding bit is off. Otherwise,
    it is on.


  - 2nd-bitmap
    The bit of 2nd-bitmap presents either a dumpable page, or not.
    If a page is on memory hole or excluded by makedumpfile command, the
    corresponding bit is off. Otherwise, it is on.


  - page header
    There are page headers corresponding to dumpable pages.
    This header presents the corresponding page information (compressed, or not.
    etc.)

    typedef struct page_desc {
        off_t                   offset;         /* the offset of the page data*/
        unsigned int            size;           /* the size of this dump page */
        unsigned int            flags;          /* flags */
        unsigned long long      page_flags;     /* page flags */
    } page_desc_t;