Re: zram OOM behavior

[Minchan Kim <minchan@kernel.org>]

On Mon, Nov 05, 2012 at 02:46:14PM +0000, Mel Gorman wrote:
> On Sat, Nov 03, 2012 at 07:36:31AM +0900, Minchan Kim wrote:
> > > <SNIP>
> > > In the first version it would never try to enter direct reclaim if a
> > > fatal signal was pending but always claim that forward progress was
> > > being made.
> > 
> > Surely we need fix for preventing deadlock with OOM kill and that's why
> > I have Cced you and this patch fixes it but my question is why we need 
> > such fatal signal checking trick.
> > 
> > How about this?
> > 
> 
> Both will work as expected but....
> 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 10090c8..881619e 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2306,13 +2306,6 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> >  
> >         throttle_direct_reclaim(gfp_mask, zonelist, nodemask);
> >  
> > -       /*
> > -        * Do not enter reclaim if fatal signal is pending. 1 is returned so
> > -        * that the page allocator does not consider triggering OOM
> > -        */
> > -       if (fatal_signal_pending(current))
> > -               return 1;
> > -
> >         trace_mm_vmscan_direct_reclaim_begin(order,
> >                                 sc.may_writepage,
> >                                 gfp_mask);
> >  
> > In this case, after throttling, current will try to do direct reclaim and
> > if he makes forward progress, he will get a memory and exit if he receive KILL signal.
> 
> It may be completely unnecessary to reclaim memory if the process that was
> throttled and killed just exits quickly. As the fatal signal is pending
> it will be able to use the pfmemalloc reserves.
> 
> > If he can't make forward progress with direct reclaim, he can ends up OOM path but
> > out_of_memory checks signal check of current and allow to access reserved memory pool
> > for quick exit and return without killing other victim selection.
> 
> While this is true, what advantage is there to having a killed process
> potentially reclaiming memory it does not need to?

Killed process needs a memory for him to be terminated. I think it's not a good idea for him
to use reserved memory pool unconditionally although he is throtlled and killed.
Because reserved memory pool is very stricted resource for emergency so using reserved memory
pool should be last resort after he fail to reclaim.

> 
> -- 
> Mel Gorman
> SUSE Labs

-- 
Kind Regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>