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

[Vivek Goyal]

On Tue, Mar 13, 2007 at 03:04:46PM -0600, Bob Montgomery wrote:
> On Wed, 2007-03-07 at 10:43 +0900, Ken'ichi Ohmichi wrote:
> 
> > 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.
> 
> Sorry you had to wait so long.  
> 
> Bob Montgomery
> 
> 

Excellent summary Bob.

> Here's what I know about how crash deals with ELF (netdump) dump files
> compared to how it deals with kdump (diskdump) dump files.
> 
> ========================================
> Intro to ELF dumpfiles and zero filling:
> ========================================
> 
> ELF format dumpfiles do not contain a page-by-page bitmap of included
> and excluded pages.  Instead, a program header table describes
> groups of contiguous pages that are present in the dumpfile.  In its
> simplest form, this allows a debugger to locate groups of pages that
> are present in the file, and conversely to identify pages that are
> missing by failing to find a program header entry that encloses the
> address of a missing page.
> 
> At some point between the 3.4 and 3.19 versions of crash, code was
> added to netdump.c:netdump_read to handle a zero_fill feature in
> the ELF files.  In a program header entry where p_memsz (MemSiz) is
> bigger than p_filsiz (FileSiz), the zone between them is considered
> to be filled with zero upon access.
> 

The same is used for handling bss where file image does not occupy space
but program image does. So it makes sense.

[..]
> ==========================
> Philosophical Meandering
> ==========================
> 
> The zero-fill feature gives an ELF dump file *three* ways to represent
> pages:
> 
> 1) Not In The Address Space: There is no program header LOAD entry
> 	that contains the requested address.
> 
> 2) Not In The File, Zero Fill: A program header LOAD entry contains
> 	the address, but the offset in the segment is between FileSiz
> 	and MemSiz.
> 
> 3) In The File: A program header LOAD entry contains the address,
> 	and the offset in the segment is smaller than FileSiz.
> 
> Unfortunately, makedumpfile recognizes *four* types of pages:
> 
> A) Not In The Address Space: physical holes in memory, undumpable zones
> B) Excluded Type: Excluded based on page type (user memory, cached page)
> C) Zero Content: Excluded based on page content being all 0x0
> D) In The File
> 
> The problem at hand is that makedumpfile's current mapping of the four
> types of pages onto ELF's three types of representation puts both
> "B) Excluded Type" and "C) Zero Content" into ELF's "2) Not In The
> File, Zero Fill" representation.  This results in crash reporting
> the contents of all addresses in excluded pages as 0x0, regardless
> of their original value.
> 

I think this is wrong. We should not be mapping excluded pages into "Not
in the File, Zero Fill". I think excluded pages should just not appear
in the program headers at all. Returning zero for an excluded page content
is not right.

> =====================================
> Can ELF Dumpfiles Solve This Problem?
> =====================================
> 
> To achieve correctness with ELF dumpfiles, one could perhaps remap the
> four types of pages to the three types of ELF representations so that
> "A) Not In The Address Space" and "B) Excluded Type" were both mapped
> to "1) Not In The Address Space".  Then "C) Zero Content" would map
> to "2) Not In The File, Zero Fill".  You would lose the ability to
> know if a page were missing because it was never in the address space
> in the first place, or because it was excluded because of its type.
> But if you read a zero, you'd know it really was a zero.
> 

I think this is the way to go. Why would I like to know if a page was
never present or mkdumpfile filtered it out? I think we can live with that
and lets just not create any sort of mapping for excluded pages in finally
generated ELF headers.

Thanks
Vivek