[PATCH] kdump : Support kernels having 64k page size.

The following  kernel patch [ along with a patch to kexec tools posted 
seperately ]
is required to generate proper core files using kdump on ppc64.

Thanks
-Sachin

On Wed, 2006-09-06 at 05:56 +0530, Sachin P. Sant wrote:

The following  kernel patch [ along with a patch to kexec tools posted 
seperately ]
is required to generate proper core files using kdump on ppc64.

Hi Sachin,

Can you provide some more explanation? It's not clear to me why this is
necessary.

thanks

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

Michael Ellerman wrote:

On Wed, 2006-09-06 at 05:56 +0530, Sachin P. Sant wrote:
 

quoted

The following  kernel patch [ along with a patch to kexec tools posted 
seperately ]
is required to generate proper core files using kdump on ppc64.
   

Hi Sachin,

Can you provide some more explanation? It's not clear to me why this is
necessary.

thanks

 

At present we are doing the backup of 32K. Thus created one ELF PT_LOAD 
segment for this region.
But, in the case of 64K page size, second segments starts at 32K and the 
first one is not page aligned.  __ioremap() (crash_dump.c) getting 
failed if pfn = 0 which is the case for the second PT_LOAD segment.  We 
did not hit this issue for 4K page size because the the first page (32K 
backup) is copied to second kernel memory and thus referencing with the 
second kernel pfn.

Here the fix is,  backup regions size is max(PAGE_SIZE,  32K)  so that 
at least one page will be part of backup ELF segment.  Drawback here is, 
we will end up 32K more for backup for 64K page size.

It can also be fixed in copy_oldmem_page() (crash_dump.c), but first 
PT_LOAD segment is not page aligned:

if (pfn > 0)
    vaddr = __ioremap(pfn << PAGE_SHIFT, PAGE_SIZE, 0);
else
    vaddr = __va(pfn << PAGE_SHIFT);


Thanks
Haren

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At present we are doing the backup of 32K. Thus created one ELF PT_LOAD 
segment for this region.
But, in the case of 64K page size, second segments starts at 32K and the 
first one is not page aligned.  __ioremap() (crash_dump.c) getting 
failed if pfn = 0 which is the case for the second PT_LOAD segment.  We 
did not hit this issue for 4K page size because the the first page (32K 
backup) is copied to second kernel memory and thus referencing with the 
second kernel pfn.

Here the fix is,  backup regions size is max(PAGE_SIZE,  32K)  so that 
at least one page will be part of backup ELF segment.  Drawback here is, 
we will end up 32K more for backup for 64K page size.

You should always do 64k regardless of the page size. I think we have
some ABI requirements here for ELF sections to be 64k aligned anyway
no ?

It can also be fixed in copy_oldmem_page() (crash_dump.c), but first 
PT_LOAD segment is not page aligned:

if (pfn > 0)
    vaddr = __ioremap(pfn << PAGE_SHIFT, PAGE_SIZE, 0);
else
    vaddr = __va(pfn << PAGE_SHIFT);

You should always do 64k regardless of the page size. I think we have
some ABI requirements here for ELF sections to be 64k aligned anyway
no ?

  

Ben are you aware of any doc where i can find more information. I 
checked the
64Bit PowerPC ELF ABI doc but couldn't find any specific information about
this.

Also other question is  If we create a 64k segment irrespective of page size
[ as compared to 32k currently ] we would be writing extra 32k even for
page size of 4K. Which means we have 32k less memory for the kdump
kernel. Wouldn't that be an issue ?

Thanks
-Sachin

On Fri, 2006-09-08 at 06:30 +0530, Sachin P. Sant wrote:

quoted

You should always do 64k regardless of the page size. I think we have
some ABI requirements here for ELF sections to be 64k aligned anyway
no ?

  

Ben are you aware of any doc where i can find more information. I 
checked the
64Bit PowerPC ELF ABI doc but couldn't find any specific information about
this.

Also other question is  If we create a 64k segment irrespective of page size
[ as compared to 32k currently ] we would be writing extra 32k even for
page size of 4K. Which means we have 32k less memory for the kdump
kernel. Wouldn't that be an issue ?

32k sounds like a drop of water in the kdump pool ...

Ben.

Benjamin Herrenschmidt wrote:

On Fri, 2006-09-08 at 06:30 +0530, Sachin P. Sant wrote:
  

quoted

quoted

You should always do 64k regardless of the page size. I think we have
some ABI requirements here for ELF sections to be 64k aligned anyway
no ?

  
      

Ben are you aware of any doc where i can find more information. I 
checked the
64Bit PowerPC ELF ABI doc but couldn't find any specific information about
this.

Also other question is  If we create a 64k segment irrespective of page size
[ as compared to 32k currently ] we would be writing extra 32k even for
page size of 4K. Which means we have 32k less memory for the kdump
kernel. Wouldn't that be an issue ?
    

32k sounds like a drop of water in the kdump pool ...

Ben.

  

The following kernel patch [ along with a patch to kexec tools posted
seperately ]is required to generate proper core files using kdump on ppc64.

* Create a backup region of 64K size irrespective of the PAGE SIZE.
 At present 32K was used as backup size. In the case of 64K page size, 
second
  PT_LOAD segments starts at 32K and the first one is not page aligned.
  __ioremap() (crash_dump.c) fails if pfn = 0 which is the case for the 
second
  PT_LOAD segment. This is not an issue for 4K page size because the the 
first
  page (32K backup) is copied to second kernel memory and thus referencing
  with the second kernel pfn.

Signed-off-by: Sachin Sant <redacted>

Thanks
-Sachin