Re: [patch] oom: thaw threads if oom killed thread is frozen before deferring

Michal Hocko
2011-09-28T10:44:51Z

On Tue 27-09-11 11:35:04, David Rientjes wrote:
> On Tue, 27 Sep 2011, Michal Hocko wrote:
>
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 626303b..c419a7e 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -32,6 +32,7 @@
> > #include <linux/mempolicy.h>
> > #include <linux/security.h>
> > #include <linux/ptrace.h>
> > +#include <linux/freezer.h>
> >
> > int sysctl_panic_on_oom;
> > int sysctl_oom_kill_allocating_task;
> > @@ -451,10 +452,15 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
> > task_pid_nr(q), q->comm);
> > task_unlock(q);
> > force_sig(SIGKILL, q);
> > +
> > + if (frozen(q))
> > + thaw_process(q);
> > }
> >
> > set_tsk_thread_flag(p, TIF_MEMDIE);
> > force_sig(SIGKILL, p);
> > + if (frozen(p))
> > + thaw_process(p);
> >
> > return 0;
> > }
>
> Also needs this...
>
>
> oom: thaw threads if oom killed thread is frozen before deferring
>
> If a thread has been oom killed and is frozen, thaw it before returning
> to the page allocator. Otherwise, it can stay frozen indefinitely and
> no memory will be freed.

OK, I can see the race now:
oom_kill_task refrigerator
set_tsk_thread_flag(p, TIF_MEMDIE);
force_sig(SIGKILL, p);
if (frozen(p))
thaw_process(p)
frozen_process();
[...]
if (!frozen(current))
break;
schedule();

select_bad_process
[...]
if (test_tsk_thread_flag(p, TIF_MEMDIE))
return ERR_PTR(-1UL);

So we either have to make sure that TIF_MEMDIE task is not frozen in
select_bad_process (your patch) or check for fatal_signal_pending
in refrigerator before we schedule and break out of the loop. Maybe the
later one is safer? Rafael?

Anyway this looks good. I guess it would be good to fold it into the
patch.

>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> mm/oom_kill.c | 5 ++++-
> 1 files changed, 4 insertions(+), 1 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -318,8 +318,11 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> * blocked waiting for another task which itself is waiting
> * for memory. Is there a better alternative?
> */
> - if (test_tsk_thread_flag(p, TIF_MEMDIE))
> + if (test_tsk_thread_flag(p, TIF_MEMDIE)) {
> + if (unlikely(frozen(p)))
> + thaw_process(p);
> return ERR_PTR(-1UL);
> + }
> if (!p->mm)
> continue;
>
>
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