Re: [PATCH] mm/oom_kill: wake futex waiters before annihilating victim shared mutex

[Michal Hocko <mhocko@suse.com>]

On Tue 07-12-21 15:47:59, Andrew Morton wrote:
> (cc's added)

Extend CC to have all futex maintainers on board.
 
> On Tue,  7 Dec 2021 16:49:02 -0500 Joel Savitz <jsavitz@redhat.com> wrote:
> 
> > In the case that two or more processes share a futex located within
> > a shared mmaped region, such as a process that shares a lock between
> > itself and a number of child processes, we have observed that when
> > a process holding the lock is oom killed, at least one waiter is never
> > alerted to this new development and simply continues to wait.
> 
> Well dang.  Is there any way of killing off that waiting process, or do
> we have a resource leak here?
> 
> > This is visible via pthreads by checking the __owner field of the
> > pthread_mutex_t structure within a waiting process, perhaps with gdb.
> > 
> > We identify reproduction of this issue by checking a waiting process of
> > a test program and viewing the contents of the pthread_mutex_t, taking note
> > of the value in the owner field, and then checking dmesg to see if the
> > owner has already been killed.
> > 
> > This issue can be tricky to reproduce, but with the modifications of
> > this small patch, I have found it to be impossible to reproduce. There
> > may be additional considerations that I have not taken into account in
> > this patch and I welcome any comments and criticism.

Why does OOM killer need a special handling. All the oom killer does is
to send a fatal signal to the victim. Why is this any different from
sending SIGKILL from the userspace?

> > Co-developed-by: Nico Pache <npache@redhat.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > Signed-off-by: Joel Savitz <jsavitz@redhat.com>
> > ---
> >  mm/oom_kill.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 1ddabefcfb5a..fa58bd10a0df 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -44,6 +44,7 @@
> >  #include <linux/kthread.h>
> >  #include <linux/init.h>
> >  #include <linux/mmu_notifier.h>
> > +#include <linux/futex.h>
> >  
> >  #include <asm/tlb.h>
> >  #include "internal.h"
> > @@ -890,6 +891,7 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
> >  	 * in order to prevent the OOM victim from depleting the memory
> >  	 * reserves from the user space under its control.
> >  	 */
> > +	futex_exit_release(victim);
> >  	do_send_sig_info(SIGKILL, SEND_SIG_PRIV, victim, PIDTYPE_TGID);
> >  	mark_oom_victim(victim);
> >  	pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB, UID:%u pgtables:%lukB oom_score_adj:%hd\n",
> > @@ -930,6 +932,7 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
> >  		 */
> >  		if (unlikely(p->flags & PF_KTHREAD))
> >  			continue;
> > +		futex_exit_release(p);
> >  		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_TGID);
> >  	}
> >  	rcu_read_unlock();
> > -- 
> > 2.33.1

-- 
Michal Hocko
SUSE Labs