task_struct.mm == NULL

task_struct.mm == NULL

[Frank Cornelis]

Hey,

I found in some code checks for task_struct.mm being NULL.
When can task_struct.mm of a process be NULL except right before the
process-kill?

Frank.

Re: task_struct.mm == NULL

[Tigran Aivazian]

Hi Frank,

It can be NULL for kernel threads which do not have a user address
space. Nevertheless, their tsk->active_mm would not be NULL but point to
some process' address space.

The point of  having active_mm is to minimize TLB flushes on switching
address spaces when the task is scheduled out.

Regards,
Tigran

On Wed, 28 Nov 2001, Frank Cornelis wrote:

> Hey,
> 
> I found in some code checks for task_struct.mm being NULL.
> When can task_struct.mm of a process be NULL except right before the
> process-kill?
> 
> Frank.
> 
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Re: task_struct.mm == NULL

[Andi Kleen]

Frank Cornelis <Frank.Cornelis@rug.ac.be> writes:

> Hey,
> 
> I found in some code checks for task_struct.mm being NULL.
> When can task_struct.mm of a process be NULL except right before the
> process-kill?

For kernel threads that run in lazy-mm mode. It allows a much cheaper context
switch into kernel threads.

-Andi

Re: task_struct.mm == NULL

[Shaya Potter]

On Wed, 2001-11-28 at 07:23, Andi Kleen wrote:
> Frank Cornelis <Frank.Cornelis@rug.ac.be> writes:
> 
> > Hey,
> > 
> > I found in some code checks for task_struct.mm being NULL.
> > When can task_struct.mm of a process be NULL except right before the
> > process-kill?
> 
> For kernel threads that run in lazy-mm mode. It allows a much cheaper context
> switch into kernel threads.

oh. so not all kernel threades have mm == null.  I used to think that
kernel threads ran in the kernel's address space, therefore there was no
point in having an mm struct as that just defines a virtual process
address space.  What's this lazy_mm mode?

thanks,

shaya