Re: [PATCH 0/2] crash: warnings of excluded page structures

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On Wed, Jan 08, 2014 at 03:32:01PM -0500, Dave Anderson wrote:
> 
> 
> ----- Original Message -----
> > 
> > 
> > 
> > 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@xxxxxxx
> 
> Cliff,
> 
> Can you make this patch far simpler?  I would prefer that an
> error message *follows* the gdb banner, i.e., where a number of
> current warnings get displayed?  You seem to be showing it just
> above the gdb banner, where it kind of gets lost.

Okay I'll move the message.
 
> Secondly, I'm not sure where/how you are determining the 1970727
> pages that are excluded.  Is it possible to put that in the
> early warning message?

The 1970727 pages were counted by the second patch. That is, I
put a counter in the readmem path that counted 'excluded' errors.
So there were 1970727 such errors during the execution of the
kmem -i.
It is probably not worth it to make a place for a new number 
in the dump header.  Knowing how many million pages were
excluded won't help if 'your' problem page was one of them.
> 
> Thirdly, I'm not convinced that the kmem locations where you
> are adding per-option warning messages are going to be the only
> place where problems may arise.  For that reason, I would
> not even bother putting them in all those various locations,
> but rather listing the "known" commands that may fail in the
> early warning message.  So do something like:

Drop the 2nd patch then.  If you think it might be more misleading
then helpful I'm fine with just the initialization warning.
> 
> $ crash vmlinux vmcore
> 
> crash 7.0.4
> Copyright (C) 2002-2013  Red Hat, Inc.
> Copyright (C) 2004, 2005, 2006, 2010  IBM Corporation
> Copyright (C) 1999-2006  Hewlett-Packard Co
> Copyright (C) 2005, 2006, 2011, 2012  Fujitsu Limited
> Copyright (C) 2006, 2007  VA Linux Systems Japan K.K.
> Copyright (C) 2005, 2011  NEC Corporation
> Copyright (C) 1999, 2002, 2007  Silicon Graphics, Inc.
> Copyright (C) 1999, 2000, 2001, 2002  Mission Critical Linux, Inc.
> This program is free software, covered by the GNU General Public License,
> and you are welcome to change it and/or distribute copies of it under
> certain conditions.  Enter "help copying" to see the conditions.
> This program has absolutely no warranty.  Enter "help warranty" for details.
>  
> GNU gdb (GDB) 7.6
> Copyright (C) 2013 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
> and "show warranty" for details.
> This GDB was configured as "x86_64-unknown-linux-gnu"...
> 
> WARNING: ominous warning message indicating (number) page structures have been
>          excluded, and that the kmem command will be affected...
> 
>       KERNEL: /usr/lib/debug/lib/modules/3.9.10-100.fc17.x86_64/vmlinux
>     DUMPFILE: /dev/crash
>         CPUS: 4
>         DATE: Wed Jan  8 15:27:14 2014
>       UPTIME: 57 days, 05:38:32
> ...
> 
> Dave
> 
> 
> 

-- 
Cliff Wickman
SGI
cpw@xxxxxxx
(651) 683-3824

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