Re: the oom killer

[Marcelo Tosatti]

On Fri, 5 Apr 2002, Andrea Arcangeli wrote:

> On Fri, Apr 05, 2002 at 01:18:26AM -0800, Andrew Morton wrote:
> > 
> > Andrea,
> > 
> > Marcelo would prefer that the VM retain the oom killer.  The thinking
> > is that if try_to_free_pages fails, then we're better off making a
> > deliberate selection of the process to kill rather than the random(ish)
> > selection which we make by failing the allocation.
> > 
> > One example is at
> > 
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=101405688319160&w=2
> > 
> > That failure was with vm-24, which I think had the less aggressive
> 
> vm-24 had a problem yes, that is fixed in the latest releases.
> 
> > i/dcache shrink code.  We do need to robustly handle the no-swap-left
> > situation.
> > 
> > So I have resurrected the oom killer.  The patch is below.
> > 
> > During testing of this, a problem cropped up.  The machine has 64 megs
> > of memory, no swap.  The workload consisted of running `make -j0
> > bzImage' in parallel with `usemem 40'.  usemem will malloc a 40
> > megabyte chunk, memset it and exit.
> > 
> > The kernel livelocked.  What appeared to be happening was that ZONE_DMA
> > was short on free pages, but ZONE_NORMAL was not.  So this check:
> > 
> > 	if (!check_classzone_need_balance(classzone))
> >         	break;
> > 
> > in try_to_free_pages() was seeing that ZONE_NORMAL had some headroom
> > and was causing a return to __alloc_pages().
> > 
> > __alloc_pages has this logic:
> > 
> > 	min = 1UL << order;
> > 	for (;;) {
> > 		zone_t *z = *(zone++);
> > 		if (!z)
> > 			break;
> > 
> > 		min += z->pages_min;
> > 		if (z->free_pages > min) {
> > 			page = rmqueue(z, order);
> > 			if (page)
> > 				return page;
> > 		}
> > 	}
> > 
> > 
> > On the first pass through this loop, `min' gets the value
> > zone_dma.pages_min + 1.  On the second pass through the loop it gets
> > the value zone_dma.pages_min + 1 + zone_normal.pages_min.  And this is
> > greater than zone_normal.free_pages! So alloc_pages() gets stuck in an
> > infinite loop.
> 
> This is a bug I fixed in the -rest patch, that's also broken on numa.
> The deadlock cannot happen if you apply all my patches.

How did you fixed this specific problem?