[Crash-utility] Kernel pages reported as PAGE_EXCLUDED

[Stefan Hajnoczi]

Hi,
I'm analyzing a dump file ("KDUMP" header) with crash 7.0.1 that seems
to have some kernel pages missing:

crash> struct foo ffff81101e19e000
struct foo struct: page excluded: kernel virtual address: ffff81101e19e000  type: "gdb_readmem_callback"
Cannot access memory at address 0xffff81101e19e000

When I set debug to 1 it turns out that even crash's 'mount' command
hits excluded pages:
mount: page excluded: kernel virtual address: ffff81101d688000  type: "read_string characters"

In order to understand this error better I took a look at
diskdump.c:read_diskdump():

  if (!page_is_dumpable(pfn)) {
          if ((dd->flags & (ZERO_EXCLUDED|ERROR_EXCLUDED)) ==
              ERROR_EXCLUDED) {
                  if (CRASHDEBUG(8))
                          fprintf(fp, "read_diskdump: PAGE_EXCLUDED: "
                              "paddr/pfn: %llx/%lx\n",
                                  (ulonglong)paddr, pfn);
                  return PAGE_EXCLUDED;
          }
          if (CRASHDEBUG(8))
                  fprintf(fp, "read_diskdump: zero-fill: "
                      "paddr/pfn: %llx/%lx\n",
                          (ulonglong)paddr, pfn);
          memset(bufptr, 0, cnt);
          return cnt;
  }

Does this mean these kernel pages are *not* zero?

Why would kernel pages containing structs (i.e. not page cache or
userspace pages) be excluded from the dump file?

(This may seem like a weird question but I did not generate the dump
file myself so I can't easily try recreating it with different options.)

Thanks,
Stefan