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

[Ken'ichi Ohmichi]

Hi Bob,

2007/03/01 12:36:01 -0700, Bob Montgomery <bob.montgomery at hp.com> wrote:
>The change in analysis behavior requires a change to crash.  Basically,
>I'd like crash to say "I don't know what is at that address" when it
>doesn't have the page in the dump instead of "That address contains a
>zero and it might be because it really was a zero and it might be
>because I don't what was there".
>
>Attached is a patch for crash (3.19) that causes it to quit providing
>zero'd pages for pages omitted from a KDUMP-style diskdump dumpfile.
>Note that this patch preserves the old behavior for DISKDUMP dumpfiles.
>(copied here for quick reference)
>
>--- crash-4.0-3.19/diskdump.c	2007-02-07 11:49:15.000000000 -0700
>+++ crash-4.0-3.19.bobm/diskdump.c	2007-03-01 11:06:54.000000000 -0700
>@@ -452,6 +452,8 @@ read_diskdump(int fd, void *bufptr, int 
> 	if ((pfn >= dd->header->max_mapnr) || !page_is_ram(pfn))
> 		return SEEK_ERROR;
> 	if (!page_is_dumpable(pfn)) {
>+		if (dd->flags & KDUMP_CMPRS_LOCAL)
>+			return SEEK_ERROR;
> 		memset(bufptr, 0, cnt);
> 		return cnt;
> 	}

In the above patch, a user cannot distinguish between the original
unaccessable pages and the excluded pages.
I think that the crash utility should display the warning message
to mean "These pages are excluded by partial dump" when it accesses 
the excluded pages.


>If a user has old KDUMP dumpfiles generated by a version of makedumpfile
>that omits zero pages and wants to continue to debug with them, one
>workaround is to "fix" those dumpfiles to work with this newer version
>of crash by changing the first 8 bytes of the dumpfile from:
>
>   "KDUMP   "
>to 
>   "DISKDUMP"
>
>Otherwise, crash would either need a command line option to allow it to
>function with old zero-excluded makedumpfile dumps, or we would need to
>distinguish the zero-fill rule in the dumpfile header ("KDUMP   " ->
>"KDUMP-NZ ") perhaps?

The existing crash cannot read a dumpfile which signature is "KDUMP-NZ ".
I think it is good that disk_dump_header.header_version will be changed
from 0 into 1. The existing crash doesn't check header_version.
If the existing crash tries to read a new dumpfile (header_version==1),
it considers the excluded pages as the zero-filled pages and it can
read a new dumpfile, and a new crash can distinguish the excluded pages.

For the above, I created the attached patch (for makedumpfile-1.1.1).
Could you say the opinion ?


>I will take a look at the behavior of crash on ELF format dumpfiles next
>week.

I appreciate your work.


Thanks
Ken'ichi Ohmichi
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