Re: [PATCH] IA64: kexec allocates too few memory for kdump kernel itself

[Jay Lan <jlan@sgi.com>]

Jay Lan wrote:
> Simon Horman wrote:
>> On Wed, Sep 03, 2008 at 02:01:59PM -0700, Jay Lan wrote:
>>> Sometimes the kexec would allocate not enough memory for kdump kernel
>>> itself on IA64 and caused kdump kernel to panic at boot.
>>>
>>> When it happens, the /proc/iomem would show a kernel RAM segment like
>>> this:
>>> 3014000000-3015294fff : System RAM
>>>   3014000000-3014823ccf : Kernel code
>>>   3014823cd0-3014dee8ef : Kernel data
>>>   3014dee8f0-301529448f : Kernel bss
>>> 3015295000-307bffdfff : System RAM
>>>   3018000000-3037ffffff : Crash kernel
>>>
>>> But kexec would allocate memory 3018000000-3019290000 for the kernel,
>>> which is 0x5000 smaller than the regular kernel. In my cases, the
>>> physical_node_map and kern_memmap of the kdump kernel overlaped and
>>> caused data corruption.
>>>
>>> This patch fixes the problem. The patch was generated against
>>> kexec-tools 2.0.0 and tested in 2.6.27-rc4.
>> Hi Jay,
>>
>> I am unclear about why this underallocation occurs.
> 
> Hi Simon,
> 
> The routine add_loaded_segments_info() set up "loaded_segment" array
> that is needed by purgatory code, based on data stored in the
> mem_ehdr array passed in as the second parameter.
> 
> Upon entrance of the routine, the crash_memory_range[] contains
> information about the regular kernel:
> crash_memory_range[ 0]: start=  3000080000, end=  30003fffff
> crash_memory_range[ 1]: start=  3003000000, end=  3005ffffff
> crash_memory_range[ 2]: start=  3006000000, end=  3013ffffff
> crash_memory_range[ 3]: start=  3014000000, end=  3015294fff
> 
> The #3 entry is the kernel memory segment.
> 
> And the mem_ehdr array would contain data as such:

Hi,

It should be mem_phdr, got it from mem_ehdr->e_phdr.

> i=0, p_paddr=3018000000, p_memsz=d04480, p_offset=10000, p_type=1
> i=1, p_paddr=3018d20000, p_memsz=9620, p_offset=d20000, p_type=1
> i=2, p_paddr=3018d30000, p_memsz=564490, p_offset=d30000, p_type=1
> i=3, p_paddr=0, p_memsz=0, p_offset=0, p_type=4

Does anyone understand how the array were created and why there
was a gap between i=0 and i=1 entries? I think this is the problem
but i do not know how to fix it, so tried to work around it.

The statement my patch replaced was totally broken:
 -			if (loaded_segments[loaded_segments_num].end !=
 -				phdr->p_paddr & ~(ELF_PAGE_SIZE-1))
 -				break;
 +			if (loaded_segments[loaded_segments_num].end <
 +			    (phdr->p_paddr & ~(ELF_PAGE_SIZE-1)) )
 +				loaded_segments[loaded_segments_num].end
 +				  = phdr->p_paddr & ~(ELF_PAGE_SIZE-1);

My debugging showed that when "loaded_segments[loaded_segments_num].end"
!= "phdr->p_paddr & ~(ELF_PAGE_SIZE-1)", they were treated as equal
and continue to next statement.  However, if i assign both expression
to local variables and do comparison, the 'break' statement is
executed correctly when two values are not the same. Unfortunately,
consequently the kdump kernel would _alawys_ hang.

I believe the intent of the original statement is to ensure there is
no gap between entries of mem_phdr array. But if there is a gap,
kexec should simply exit with failure. The 'break' statement just
created a loaded_segment[] array that broke the kernel memory segment
into multiple entries and resulted in the kdump kernel hang in
find_memory(). The IA64 (at least 2.6.27-rc4) kdump kernel works in
some cases today are simply out of luck.

I believe the real fix is to fix the contents of the mem_phdr array.
Since i do not know how to fix it, my patch would close up the
gap where there is the a gap between entries of the mem_phdr array.

Does it make more sense to you now, Simon?

Regards,
 - jay



> 
> The code wants the new loaded_segments contain starting address
> all aligned at page boundary, which is 0x10000 in IA64.
> 
> Note that the p_memsz of mem_ehdr does not match to entries in
> /proc/iomem:
> 3014000000-3015294fff : System RAM
>   3014000000-3014823ccf : Kernel code
>   3014823cd0-3014dee8ef : Kernel data
>   3014dee8f0-301529448f : Kernel bss
> 
> The original code of add_loaded_segments_info() would go through
> the mem_ehdr array and use the p_paddr of the first entry (the
> beginning of the reserved memory) as the start address, add
> the p_memsz of three entries to calculate the end address of
> the kernel segment.
> 
> But the p_paddr of i=0 plus p_memsz of i=0 should result in
> 3018d10000 as the p_paddr of i=1 entry, but actually the
> p_paddr of i=1 is 3018d20000. The logic of that routine
> can not explain the discrepency.
> 
> So, where the data of mem_ehdr array come from?
> 
> add_loaded_segments_info
> <-  load_crashdump_segments
>   <-  elf_ia64_load
>     <- file_type[i].load
>       <- my_load
> 
> The elf_ia64_load set up mem_ehdr, probabaly based on data
> pointed by *buf, which i think comes from vmlinuz.
> 
> So, i failed to find out how the p_memsz were set up initially.
> But, i think we did it the way too complicated, IMHO.
> 
> The crash_memory_range[] array showed the kernel segment consumed
> 0x1295000 bytes of memory and we only need to tell the purgatory
> code to reserve that amount of memory. The logic in
> add_loaded_segments_info() came out with 0x1290000 and caused the
> crashkernel to panic on boot.
> 
> Hmmm, as i types now, i may not consider the situation where
> the crashkernel is not the same as the first kernel...
> 
> Note that the underallocation does not _ALWAYS_ happen! It depends
> on the vmlinux we build. Honestly i do not understand some part of
> the kexec-tools code well enough to make major surgery to the code.
> So, i just compare the end address after calculation of i=0 entry
> of mem_ehdr array with the start address of the second entry. If it
> is too small, i just bring it up to align with the start address of
> the second entry. I am happy to allocate one extra page, may not be
> needed in some cases, of memory than to panic. Yes, my patch is
> a work-around.
> 
> If you can find the true cause of the problem and fix it, it
> would be great and appreciated!
> 
> Regards,
>  - jay
> 
> 
>>> Signed-off-by: Jay Lan <jlan@sgi.com>
>>>
>>> ---
>>>  kexec/arch/ia64/crashdump-ia64.c |    8 ++++----
>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> Index: kexec-tools/kexec/arch/ia64/crashdump-ia64.c
>>> ===================================================================
>>> --- kexec-tools.orig/kexec/arch/ia64/crashdump-ia64.c	2008-09-03 11:24:14.289758063 -0700
>>> +++ kexec-tools/kexec/arch/ia64/crashdump-ia64.c	2008-09-03 11:29:34.095833316 -0700
>>> @@ -90,15 +90,15 @@ static void add_loaded_segments_info(str
>>>  			phdr = &ehdr->e_phdr[i];
>>>  	                if (phdr->p_type != PT_LOAD)
>>>  	                        break;
>>> -			if (loaded_segments[loaded_segments_num].end !=
>>> -				phdr->p_paddr & ~(ELF_PAGE_SIZE-1))
>>> -				break;
>>> +			if (loaded_segments[loaded_segments_num].end <
>>> +			    (phdr->p_paddr & ~(ELF_PAGE_SIZE-1)) )
>>> +				loaded_segments[loaded_segments_num].end
>>> +				  = phdr->p_paddr & ~(ELF_PAGE_SIZE-1);
>>>  			loaded_segments[loaded_segments_num].end +=
>>>  				(phdr->p_memsz + ELF_PAGE_SIZE - 1) &
>>>  				~(ELF_PAGE_SIZE - 1);
>>>  			i++;
>>>  		}
>>> -
>>>  		loaded_segments_num++;
>>>  	}
>>>  }
> 
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