----- 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