DEBUG_PAGEALLOC on PPC not working (kernels 2.6-25, 3.0-34)

[Scott Wood]

On 06/20/2013 05:42:40 AM, perth1415 wrote:
> Hi Scott,
> 
> Thanks for the reply, though a bit disheartening :-)
> My understanding on e500 MMU is not clear. It'd be nice if I could  
> find some
> way (may be ad-hoc) to debug some use-after-free page corruptions.  
> SLAB
> debug tells me the page was modified by someone after it was freed but
> DEBUG_PAGEALLOC would have been more specific, as to tell me where  
> exactly
> it was getting modified.
> Any debugging clues will be much appreciated.

If you know an exact address that's being corrupted, you could set a  
data breakpoint (by manually setting the registers, and making sure  
that the exception handler will produce a dump and not ignore it as a  
spurious event).  You could add code to periodically check for  
corruption (from a timer, from codepaths which you suspect,  
before/after IRQ handlers, etc).  If you have specific code that you  
suspect may be responsible, you can have it check for poison values  
before writing.  I'm not sure if slab debugging already does this, but  
if not you could have it record the address of the code that last  
allocated and freed the corrupted memory chunk.

If you have access to a tool such as Virtutech Simics, you could use  
reverse execution to find the corruption.

Or you could find a way to separate the code/data needed by exceptions  
(including page tables, kernel stacks, etc) from everything else, and  
only pin the former, but that's probably a lot of work.

-Scott