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

[Takao Indoh]

Hi Bob, Dave,

On Wed, 08 Nov 2006 16:08:08 -0500, Dave Anderson wrote:

>Bob Montgomery wrote:
>
>> I've been experimenting with the makedumpfile utility for kdump on ia64.
>> One of my experiments was to verify that a page that should have been
>> missing indeed was missing.  I used crash 4.0-3.8 to look for a user
>> page that should have been omitted from the dump.
>>
>> crash> x/xg 0xe0000040fc00c000
>> 0xe0000040fc00c000:     0x0000000000000000
>>
>> On a full dump from makedumpfile as well as on a straight copy of
>> vmcore, crash reports this:
>>
>> crash> x/xg 0xe0000040fc00c000
>> 0xe0000040fc00c000:     0x00010102464c457f
>>
>> The dumpfiles created by makedumpfile appear to crash as diskdump files,
>> and crash appears to excuse missing pages and report 0x0 contents here:
>>
>> diskdump.c:read_diskdump, line 454:
>>
>>        if (!page_is_dumpable(pfn)) {
>>                 memset(bufptr, 0, cnt);
>>                 return cnt;
>>
>> Shouldn't there be some indication that a requested page is missing as
>> opposed to being legitimately full of zeros?
>>
>> Bob Montgomery
>
>Hi Bob,
>
>That decision was made by the diskdump developers
>when I accepted their diskdump-specific work into the
>crash utility.
>
>As I recall, I kind of agreed with you at the time, but there
>was some compelling reason that they had that argued for
>passing a zero-filled page -- although I don't recall what it
>was?
>
>I don't believe it had anything to do with user pages,
>but in the case of some of the other kernel pages that can
>be left out of the dump, passing back a zero-filled page
>was a cleaner way to handle whatever the problem was.
>
>Anyway, I've long since forgotten why they wanted it
>that way...
>
>Perhaps the diskdump guys on this list can refresh my
>memory?

My memory is also unclear...
As far as my dim memory goes, I decided the following policy when I 
wrote this code.

1) If the page is not found because of invalid address(e.g. address
   points memory hole), read_diskdump should return error.
2) If the page is not found because of partial dump, read_diskdump
   should return zero-filled page.

Perhaps I decided the 2nd policy because of simplicity of implementation.
Bob points out that crash should do something instead of passing
zero-filled page (e.g. showing message) in the 2nd case, right?

Takao Indoh