[Xen-devel] x86-64 problem with invalid page fault in linux 2.6.16-rc1

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

I'm just trying to see if anyone has any clue about this, which only appears to 
happen with MP guests:

Since the check for the modules area is gone in 2.6.16's vmalloc_fault() (and 
we appropriately merged this change to
the Xen files), we are now seeing page faults in the module area, where a 
subsequent software page table walk shows all
page table entries present, and a get_user from inside the hypervisor's or the 
guest's page fault handler also succeeds.
The module in the questionable space was loaded significantly before the page 
fault occurs, and we never saw a fault
after the system fully booted. Faults of this kind may have existed before, but 
would have been hidden by the
vmalloc_fault() handling assuming that another processor would have put in 
place the pgd entry meanwhile.

Since I have no clue how such a fault could be raised in the first place (given 
that the pgd entry for the modules area
is shared with main kernel code, all lower level entries are shared across 
kernel and all processes, and the fault
happens on an access to the modules area from kernel code [guaranteeing that 
the system isn't unintentionally running
with the user mode page tables]).

I can also mostly rule out any sort of hardware problem since the issue is 
visible on both Intel and AMD processors.

Thanks for any thoughts, hints, or pointers,
Jan

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