[Crash-utility] handling missing kdump pages in diskdump format

[Ken'ichi Ohmichi]

Hi Vivek,

2007/03/06 12:02:46 +0530, Vivek Goyal <vgoyal at in.ibm.com> wrote:
>On Thu, Mar 01, 2007 at 09:15:05PM +0900, Ken'ichi Ohmichi wrote:
>> 
>> (added Vivek Goyal)
>> (added redhat-kexec-kdump-ml)
>> 
>> Hi everyone,
>> 
>> If Bob's proposal (the end of this mail) is merged into makedumpfile,
>> the analysis behavior of ELF dumpfile is different from kdump-compressed
>> dumpfile's as follows.
>> When reading ELF dumpfile, the crash utility treats the excluded pages
>> as zero-filled pages. But, as to kdump-compressed dumpfile, the crash
>> utility will display the warning message to mean "These pages are
>> excluded by partial dump" when it accesses the excluded pages.
>> 
>
>I quickly went through this thread. I don't know much about the diskdump
>format but from a layman's perspective, instead of keeping the zero pages
>and compressing them, why not extend diskdump format and maintain another
>bitmap which signifies the valid zero pages but they are not physically 
>part of the core file? I think overall it might reduce dump size.

Thank you for the comment.
I think that it is unnecessary to extend diskdump format.
Instead of having all the compressed zero-pages, it is enough that
page descriptors of all zero-pages point same zero-page.
In this implement, the size of dumpfile increases by only one page
(4K, 16K, etc.). The attached patch is for it.


>Secondly, for ELF format core files, probably we can disable the behavior of
>excluding zero filled pages from dumpfile and then "crash" behavior can
>be consistent. (Return zero filled page only for valid data otherwise crib).

I want to use the feature of excluding zero-pages, because our systems
(x86_64) have many zero-pages immediately after system booting.
Bob is researching for the behavior of crash on ELF format dumpfiles.
I would like to wait for his report.


Thanks
Ken'ichi Ohmichi

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