context overflow

[Dan Malek]

tom_gall at vnet.ibm.com wrote:

>   current->mm I believe is correct. active_mm for tasks in user space just point
> back to mm. kernel space tasks will have an mm of NULL yet their active_mm will
> point back to the last user space task they ran.


Not exactly.  Every task running on a CPU must have an active_mm, and
it represents the current context for the MMU.  This active_mm comes
from a single threaded application's 'mm', or in the case of a
thread without an 'mm' from the previous application that ran, or
from somewhere else depending upon VM_CLONE games.

The point you are missing is 'active_mm' represents the current
context for the MMU.  If you get a context overflow, you can't skip
getting and setting a context for an active task just because it
doesn't have a 'current->mm'.  Your modification to do this
results in a task running on a CPU with a "NO CONTEXT" mm, and worse
and incorrect VSID/ASID/PID/whatever for the task running on that MMU.


>   The reason for this patch is in the case where the idle task comes in on one
> processor and on another processor it has encountered a context overflow.


It's not just the idle task.  It could be any task that is supposed
to get an active_mm from someone else.

The patch is just logically incorrect.  There should be no
'if current->mm' and it should get/set context on current->active_mm.


	-- Dan

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