Re: [PATCH 1/5] mm: Add __GFP_NO_OOM_KILL flag

[Andrew Morton]

On Thu, 7 May 2009 15:16:17 -0700 (PDT)
David Rientjes <rientjes@xxxxxxxxxx> wrote:

> On Thu, 7 May 2009, Andrew Morton wrote:
> 
> > - the standard way of controlling memory allocator behaviour is via
> >   the gfp_t.  Bypassing that is an unusual step and needs a higher
> >   level of justification, which I'm not seeing here.
> > 
> 
> The standard way of controlling the oom killer behavior for a zone is via 
> the ZONE_OOM_LOCKED bit.

oop, I didn't remember/realise that ZONE_OOM_LOCKED already exists.

> > - if we do this via an unusual global, we reduce the chances that
> >   another subsytem could use the new feature.
> > 
> >   I don't know what subsytem that might be, but I bet they're out
> >   there.  checkpoint-restart, virtual machines, ballooning memory
> >   drivers, kexec loading, etc.
> > 
> 
> There's two separate issues here: the use of ZONE_OOM_LOCKED to control 
> whether or not to invoke the oom killer for a specific zone (which is 
> already its only function), and the fact that in this case we're doing it 
> for all zones.  It seems like you're concerned with the latter, but the 
> distinction in the hibernation case is that no memory freeing would be 
> possible as the result of the oom killer for _all_ zones, so it makes 
> sense to lock them all out.

OK.

> > > The fact is that _all_ allocations here are implicitly __GFP_NO_OOM_KILL 
> > > whether it specifies it or not since the oom killer would simply kill a 
> > > task in D state which can't exit or free memory and subsequent allocations 
> > > would make the oom killer a no-op because there's an eligible task with 
> > > TIF_MEMDIE set.  The only thing you're saving with __GFP_NO_OOM_KILL is 
> > > calling the oom killer in a first place and killing an unresponsive task 
> > > but that would have to happen anyway when thawed since the system is oom 
> > > (or otherwise lockup for GFP_KERNEL with order < PAGE_ALLOC_COSTLY_ORDER).
> > 
> > All the above is specific to the PM application only, when userspace
> > tasks are stopped.
> > 
> 
> I'm not arguing that the only way we can ever implement __GFP_NO_OOM_KILL 
> is for the entire system: we can set ZONE_OOM_LOCKED for only the zones in 
> the zonelist that are passed to the page allocator.  For this particular 
> purpose, that is naturally all zones; for other future use cases it may be 
> chosen only to lock out the zones we're allowed to allocate from in that 
> context.

OK.

> > It might well end up that stopping userspace (beforehand or before
> > oom-killing) is a hard requirement for reliably disabling the
> > oom-killer.
> 
> Yes, globally, but future use cases may disable only specific zones such 
> as with memory hot-remove.

<goes off to find out what ZONE_OOM_LOCKED does>

That took remarkably longer than one would have expected..

Yes, OK, I agree, globally setting ZONE_OOM_LOCKED would produce a
decent result.

The setting and clearing of that thing looks gruesomely racy..

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