Re: [patch 3/3 -mmotm] oom: invoke oom killer for __GFP_NOFAIL

[Peter Zijlstra]

On Tue, 2009-06-02 at 00:26 -0700, David Rientjes wrote:
> > I really think/hope/expect that this is unneeded.
> > 
> > Do we know of any callsites which do greater-than-order-0 allocations
> > with GFP_NOFAIL?  If so, we should fix them.
> > 
> > Then just ban order>0 && GFP_NOFAIL allocations.
> > 
> 
> That seems like a different topic: banning higher-order __GFP_NOFAIL 
> allocations or just deprecating __GFP_NOFAIL altogether and slowly 
> switching users over is a worthwhile effort, but is unrelated.
> 
> This patch is necessary because we explicitly deny the oom killer from 
> being used when the order is greater than PAGE_ALLOC_COSTLY_ORDER because 
> of an assumption that it won't help.  That assumption isn't always true, 
> especially for large memory-hogging tasks that have mlocked large chunks 
> of contiguous memory, for example.  The only thing we do know is that 
> direct reclaim has not made any progress so we're unlikely to get a 
> substantial amount of memory freeing in the immediate future.  Such an 
> instance will simply loop forever without killing that rogue task for a 
> __GFP_NOFAIL allocation.
> 
> So while it's better in the long-term to deprecate the flag as much as 
> possible and perhaps someday remove it from the page allocator entirely, 
> we're faced with the current behavior of either looping endlessly or 
> freeing memory so the kernel allocation may succeed when direct reclaim 
> has failed, which also makes this a rare instance where the oom killer 
> will never needlessly kill a task.

I would really prefer if we do as Andrew suggests. Both will fix this
problem, so I don't see it as a different topic at all.

Eradicating __GFP_NOFAIL is a fine goal, but very hard work (people have
been wanting to do that for many years). But simply limiting it to
0-order allocation should be much(?) easier.


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