On Fri, Mar 17, 2023 at 10:26 AM HAGIO KAZUHITO(萩尾　一仁) <k-hagio-ab@nec.com> wrote:
On 2023/03/17 11:12, lijiang wrote:
> Hi, Kazu
> Thank you for the comment.
> On Fri, Mar 17, 2023 at 9:02 AM HAGIO KAZUHITO(萩尾　一仁) <k-hagio-ab@nec.com <mailto:k-hagio-ab@nec.com>> wrote:
>
>     Hi Lianbo,
>
>     On 2023/03/15 18:07, Lianbo Jiang wrote:
>      > The help/man page of the "vm" command suggests that the "-M" option
>      > accepts the mm_struct address as a valid argument. However, the "vm
>      > -M" option always prints the virtual memory data of the current
>      > task, regardless of its arguments.
>
>     I think that it's the intended behavior, isn't it?
>
>                -M mm  if the mm_struct address has been removed from the task_struct
>                       of an exiting task, the virtual memory data cannot be displayed.
>                       However, if the address can be determined from the kernel stack,
>                       it can be entered manually in order to try to resurrect the
>                       virtual memory data of the task.
>
> They are not the same issues.
>
> For the live debugging, take the sshd as an example:
>
> Without the patch:
> crash> ps |grep sshd
>       1159       1   3  ffff9915858a9980  IN   0.1    15820     9020  sshd
> 51573    1159   2  ffff991572868000  IN   0.1    18916    11168  sshd
>      51595   51573   2  ffff991575026600  IN   0.0    18784     6892  sshd
> crash> vm -M ffff991572868000
> PID: 51727    TASK: ffff9914465cb300  CPU: 0    COMMAND: "crash"   <------not a correct result

No, I meant that this is correct.  Let me explain,

(1) The "-M mm" option accepts an *mm_struct* address.
(2) The specified mm is enabled only when the mm_struct address has been
removedfrom the task_struct, i.e. mm = 0.


Thanks for your explanation, Kazu.

Let's consider the following two cases:
[1] crash> vm pid -M mm
[2] crash> vm -M mm

For the case [1], it is expected usage, and requires that the given arguments must satisfy the above (1) and (2). The documentation needs to clarify this to avoid misunderstanding. It might be necessary to update the man page.

For the case [2], if the usage of "vm -M mm" is not allowed, the "vm -M mm" needs to output an error instead of always displaying a bogus result.

But anyway, they are confusing. We can not assume that crash tools will always get the correct usage or arguments. Any thoughts?

Thanks
Lianbo
 
So we can emulate that situation e.g. with kthreadd (pid=2) like this:

crash> vm 1
PID: 1        TASK: ffff900b41599840  CPU: 11   COMMAND: "systemd"
        MM               PGD          RSS    TOTAL_VM
ffff9019ff0e3f00  ffff9019ffc10000  12516k  250328k
       VMA           START       END     FLAGS FILE
ffff900c94de2488 55e2c48d1000 55e2c4a13000 8000875 /usr/lib/systemd/systemd
...

crash> vm 2
PID: 2        TASK: ffff900b4159b080  CPU: 2    COMMAND: "kthreadd"
        MM               PGD          RSS    TOTAL_VM
        0                 0            0k       0k
       ^^^
crash> vm 2 -M ffff9019ff0e3f00  <<-- systemd's mm
PID: 2        TASK: ffff900b4159b080  CPU: 2    COMMAND: "kthreadd"
        MM               PGD          RSS    TOTAL_VM
        0                 0            0k       0k
       VMA           START       END     FLAGS FILE
ffff900c94de2488 55e2c48d1000 55e2c4a13000 8000875 /usr/lib/systemd/systemd

I think this is the expected usage of the "-M mm" option.

Thanks,
Kazu