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

[Dave Anderson]

----- Original Message -----
> On 12/08/2014 10:28 PM, Dave Anderson wrote:
> >
> >
> > ----- Original Message -----
> >> 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.
> >
> > Well, the vm_area_dump() function is relatively stable, so let's not go crazy
> > here for what's essentially an "experimental" option.
> 
> I see.
> 
> >
> >>
> >>>
> >>> 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.
> >
> > Actually it doesn't take much work at all.  If both tc->mm_struct and tm->mm_struct_addr
> > are replaced with the supplied address:
> >
> >       tc->mm_struct = tm->mm_struct_addr = pc->curcmd_private;
> >
> > then "vm -M ffff880495120dc0 -p" also works OK with my sample vmcore.
> 
> Yes, vm -p will tc->mm_struct to get pgd. But I was afraid permanently changing tc->mm_struct
> is not good.
> 
> Take my core into consideration, the case is:
> 
> <cut>
> crash> help -t | grep mm_struct
>          .mm_struct: 0
>           mm_struct: 354cc80
> crash> vm -M 0xffff88003ae98ac0
> PID: 4860   TASK: ffff88003ae7eaf0  CPU: 0   COMMAND: "bash"
>         MM               PGD          RSS    TOTAL_VM
>         0                 0            0k       0k
>        VMA           START       END     FLAGS FILE
> ffff88003acc3ad8     400000     4d4000 8001875 /bin/bash
> ...
> crash> help -t | grep mm_struct
>          .mm_struct: ffff88003ae98ac0
>           mm_struct: 354cc80
> crash>
> <cut>
> 
> Is it OK to have ".mm_struct" changed here?

Probably not...

I did take a quick look at the other usages of tc->mm_struct, and I think
the only major difference is that the "search -u" command would search
the user-space memory of the changed task.  But there's also an oddball
check for an i386 hypervisor callback from user-space, and it looks like
get_task_mem_usage() would use it from that point on, so I agree with
you that it should be restored to NULL.  

Since there's a strong possibility of an error(FATAL, ...) call while
executing the command, it's not safe to simply restore it to NULL at the
end of the command, but rather the pc->cmd_cleanup() facility should
be used with the task address as the pc->cmd_cleanup_arg. 

> >
> >>>
> >>> 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.
> >
> > I suppose you could, although in all probability it's going to be stay in the mm_struct
> > slab cache, and worst case, have been re-used by another task.
> 
> So you like the modification.

Sure -- with the pc->cmd_cleanup in place, I don't see how it can hurt.

Dave