[Crash-utility] [PATCH 0/2] crash: warnings of excluded page structures

[cpw]

I have proposed a patch to makedumpfile to (optionally) exclude from
a dump the page structures representing excluded pages.
The idea being that a system with terabytes of system memory has
millions of pages of page structures.  And most of them are unneeded.

That patch thread begins here:
  http://marc.info/?l=kexec&m=138853299130125&w=2

Dave [Anderson] raised these crash-related issues;
  Although I'm sure you tested this, I find it amazing that
  only the "kmem -[fF]" option is the only command option
  that is affected?
  If I'm not mistaken, this would be the first time that legitimate
  kernel data would be excluded from the dump, and the user would
  have no obvious way of knowing that it had been done, correct?
  If it were encoded in the in the header somewhere, at least a
  warning message could be printed during crash initialization.
  ...
  Right, but look at all of the other page struct offsets in addition to
  page.lru that are used.  The page.flags usage comes to mind, and for
  example, what would "kmem -p" display for the missing pages?
  Or "kmem <address>"?  And would "kmem -i" display invalid data?
  I'm just speculating off the top of my head, but the page structure is
  such a fundamental data structure with several of its fields being used,
  just enter "help -o page_" to see all of its potential member usages.

So I am submitting two patches for your consideration, should the patch
to exclude unused vmemmap pages be taken into makedumpfile.

- [PATCH 1/2] crash: initial note of excluded page structures
  This one makes crash startup look like this:
     This program has absolutely no warranty.  Enter "help warranty" for details.
 
     NOTE: Unused vmemmap page structures are excluded from this dump.
     GNU gdb (GDB) 7.6
     Copyright (C) 2013 Free Software Foundation, Inc.

- [PATCH 1/2] crash: kmem warnings for excluded page structures
   This patch modifies kmem options -f, -F, -s addr, -S addr, and -i.
   Those are the only options that I could detect looking for excluded pages.

   This patch applies on top of the first, and adds some warnings to the
   output of these kmem options. For example:

    crash> kmem -f
    Note: kmem -f may fail because unused page structures are excluded from this dump.
    NODE
      0
    ZONE  NAME        SIZE    FREE      MEM_MAP       START_PADDR  START_MAPNR
      0   DMA         4095    3934  ffffea0000000038      1000          0     
    AREA    SIZE  FREE_AREA_STRUCT  BLOCKS  PAGES
      0       4k  ffff880000013068       2      2
     ...

   crash> kmem -i
                 PAGES        TOTAL      PERCENTAGE
    TOTAL MEM  128008147     488.3 GB         ----
         FREE  127599276     486.8 GB   99% of TOTAL MEM
         USED   408871       1.6 GB    0% of TOTAL MEM
       SHARED    11049      43.2 MB    0% of TOTAL MEM
      BUFFERS     5722      22.4 MB    0% of TOTAL MEM
       CACHED    44638     174.4 MB    0% of TOTAL MEM
         SLAB    62139     242.7 MB    0% of TOTAL MEM
   
   TOTAL SWAP  4893032      18.7 GB         ----
    SWAP USED        0            0    0% of TOTAL SWAP
    SWAP FREE  4893032      18.7 GB  100% of TOTAL SWAP
   
   Note: 1970727 free pages not found (excluded); results are incomplete.
   Unused page structures are excluded from this dump.

-Cliff Wickman
 cpw at sgi.com