Re: [bug report] OOM caused by mlock() during exiting blktrace

Hello Jeff,

On Wed, Mar 24, 2021 at 11:07 PM Jeff Moyer <[email protected]> wrote:
>
> Hi, Ming,
>
> Ming Lei <[email protected]> writes:
>
> > Hello Guys,
> >
> > It is observed that nr_cpu * mlock(128M) is called when exiting
> > 'blktrace -b 8192 -d $DEV'. And OOM is easily triggered when the
> > machine has lots of CPU cores.
>
> The only mlock in the code appears to happen at setup time.  How did you
> determine this was occurring in the exit path?

It is triggered when pressing 'ctrl^ + C' on 'blktrace -b 8192 -d /dev/nullb0',
at that time blktrace needs to parse collected trace, even though there is
nothing collected on nullb0.

>
> > So can we figure out one way to not lock big buffer for avoiding
> > OOM?
>
> The obvious question is why are you using -b 8192?  Were you losing
> events without that option?

It is used by device-mapper[1] which is covered in Red Hat kernel CKI.

Seems the locked buffer size is 16 * (buffer size) which is specified by '-b'.
So at default per-cpu mlock(16 * 512K) will be done, when nr_cpu is bigger,
it still risks in OOM.

[1] https://github.com/jthornber/device-mapper-test-suite.git


Thanks,
Ming Lei