[PATCH] lowmemorykiller: prevent multiple instances of low memory killer

[David Rientjes]

On Tue, 16 Apr 2013, Oskar Andero wrote:

> > > The comment in shrinker.h is misleading, not the source code.
> > > do_shrinker_shrink() will fail for anything negative and 0.
> > 
> > The comment is correct.  The only acceptable negative return is -1.
> > Look at the second time do_shrinker_shrink() is called from
> > shrink_slab().
> > 
> >    283                  while (total_scan >= batch_size) {
> >    284                          int nr_before;
> >    285  
> >    286                          nr_before = do_shrinker_shrink(shrinker, shrink, 0);
> >    287                          shrink_ret = do_shrinker_shrink(shrinker, shrink,
> >    288                                                          batch_size);
> >    289                          if (shrink_ret == -1)
> >    290                                  break;
> >    291                          if (shrink_ret < nr_before)
> >    292                                  ret += nr_before - shrink_ret;
> >    293                          count_vm_events(SLABS_SCANNED, batch_size);
> 
> Yes, the comment is correct with what is implemented in the code, but
> that doesn't mean the code is right. IMHO, relaying on magical numbers is highly
> questionable coding style.
> 

No, it's not.  This is controlled higher in shrink_slab() by this:

	max_pass = do_shrinker_shrink(shrinker, shrink, 0);
	if (max_pass <= 0)
		continue;

and your patch is implemented incorrectly, i.e. it does not return 
LMK_BUSY if the spinlock is contended which needlessly recalls the 
shrinker later.

You have a couple of options:

 - return -1 when the spinlock is contended immediately when
   !sc->nr_to_scan (although it should really be a cmpxchg since a
   spinlock isn't needed), or

 - protect the for_each_process() loop in lowmem_shrink() with an
   actual spinlock that will detect any previously killed process
   since it will have the TIF_MEMDIE bit set.