[Crash-utility] About displaying virtual memory information of exiting task

[qiaonuohan]

On 12/06/2014 04:11 AM, Dave Anderson wrote:
> Interestingly enough, today I was asked to look at a vmcore in which an oops
> occurred during task exit after tsk->mm had been NULL'd out in exit_mm():

It almost matches what I am facing, when tsk->mm is set to NULL and memory
mapping is supposed to be displayed. This is a more simple implementation.
I have tried to command like vm [taskp | pid | [-M mm_struct]]. But it have
to modify a lot of thing.

By the way, I feel the code is becoming more and more complicated, maybe a
reconstruction is needed.

>
> Of course it has its limitations.  Since the page tables are being broken down in this case,
> "vm -p" fails:
>
>   crash>  vm -M ffff880495120dc0 -p
>    PID: 4563   TASK: ffff88049863f500  CPU: 8   COMMAND: "postgres"
>           MM               PGD          RSS    TOTAL_VM
>           0                 0            0k       0k
>          VMA           START       END     FLAGS FILE
>    ffff8804a085ce90     400000     f56000 8001875 /usr/local/greenplum-db-4.3.3.1/bin/postgres
>    VIRTUAL     PHYSICAL
>    vm: invalid kernel virtual address: 50  type: "mm_struct pgd"
>    crash>

After a glance, the pgd comes from the mm of task_struct. We need a lot of work to make it
replaced by argument of -M, I don't think it worse it right now.

>
> But it does seems like a worthwhile addition.
>
> The patch doesn't check whether mm->owner or mm->mm_count are legitimate, but I'm not
> sure whether it's even worth it?  If it fails, it fails, and the help page should just
> indicate that the command option is not guaranteed to work.  Does the attached patch work
> for you?

Similar to the core I got. And I modified the patch to add some check. At least I think
we need to make sure the address still belongs to a mm_struct object.

-- 
Regards
Qiao Nuohan
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