Re: [Crash-utility] About displaying virtual memory information of exiting task
----- Original Message -----
Hello Dave,
I'd like to discuss about the following feature with you and get some advise.
vm command is used to display virtual memory information of a task. But if the
task is exiting(according to crash, 'tsk->flags & PF_EXITING' is true), vm
will
set mm to 0 in get_task_mem_usage(). But the mm may be not freed yet, the mm and
its related virtual information is helpful when debuging a exiting task.
But it may have been freed, and in the case of CONFIG_SLUB, the mm_struct.mmap member
would be overwritten as a free slab object link pointer, making it useless. Or it
could have been freed-and-reused.
I was considering to ignore the IS_EXITING(task) in get_task_mem_usage() and
if tsk->mm is set to NULL but the mm is not freed(see the following case), then we
can specify the mm manually.
CASE(the code is from kernel):
<cut>
exit_mm()
{
...
tsk->mm = NULL; --> dump after this, and before mmput() freeing mm
...
mmput(mm);
}
<cut>
But I guess it is not a good design to you. So I reconsidered it. What about specifying
mm to vm just like task's pid or address. Then vm can retrieve virtual memory
information
from specified mm directly. And get the owner task from mm->owner.
That might work, at least if:
(1) the mm_struct has not been freed (SLUB),
(2) the mm_struct has not been freed-and-reused, and
(3) the kernel is configured with CONFIG_MEMCG and mm->owner points to the exiting
task.
But how would a typical user of this option know what the mm_struct address is?
Dave