Re: [15/17] SLUB: Support virtual fallback via SLAB_VFALLBACK

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 29 Sep 2007 10:47:12 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> 
> On Sat, 2007-09-29 at 01:13 -0700, Andrew Morton wrote:
> > On Fri, 28 Sep 2007 20:25:50 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > 
> > > 
> > > On Fri, 2007-09-28 at 11:20 -0700, Christoph Lameter wrote:
> > > 
> > > > > start 2 processes that each mmap a separate 64M file, and which does
> > > > > sequential writes on them. start a 3th process that does the same with
> > > > > 64M anonymous.
> > > > > 
> > > > > wait for a while, and you'll see order=1 failures.
> > > > 
> > > > Really? That means we can no longer even allocate stacks for forking.
> > > > 
> > > > Its surprising that neither lumpy reclaim nor the mobility patches can 
> > > > deal with it? Lumpy reclaim should be able to free neighboring pages to 
> > > > avoid the order 1 failure unless there are lots of pinned pages.
> > > > 
> > > > I guess then that lots of pages are pinned through I/O?
> > > 
> > > memory got massively fragemented, as anti-frag gets easily defeated.
> > > setting min_free_kbytes to 12M does seem to solve it - it forces 2 max
> > > order blocks to stay available, so we don't mix types. however 12M on
> > > 128M is rather a lot.
> > > 
> > > its still on my todo list to look at it further..
> > > 
> > 
> > That would be really really bad (as in: patch-dropping time) if those
> > order-1 allocations are not atomic.
> > 
> > What's the callsite? 
> 
> Ah, right, that was the detail... all this lumpy reclaim is useless for
> atomic allocations. And with SLUB using higher order pages, atomic !0
> order allocations will be very very common.

Oh OK.

I thought we'd already fixed slub so that it didn't do that.  Maybe that
fix is in -mm but I don't think so.

Trying to do atomic order-1 allocations on behalf of arbitray slab caches
just won't fly - this is a significant degradation in kernel reliability,
as you've very easily demonstrated.

> One I can remember was:
> 
>   add_to_page_cache()
>     radix_tree_insert()
>       radix_tree_node_alloc()
>         kmem_cache_alloc()
> 
> which is an atomic callsite.
> 
> Which leaves us in a situation where we can load pages, because there is
> free memory, but can't manage to allocate memory to track them.. 

Right.  Leading to application failure which for many is equivalent to a
complete system outage.

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