Re: [PATCH 0/5] make slab gfp fair

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2007-05-14 at 12:44 -0700, Andrew Morton wrote:
> On Mon, 14 May 2007 11:12:24 -0500
> Matt Mackall <mpm@selenic.com> wrote:
> 
> > If I understand this correctly:
> > 
> > privileged thread                      unprivileged greedy process
> > kmem_cache_alloc(...)
> >    adds new slab page from lowmem pool
> > do_io()
> >                                        kmem_cache_alloc(...)
> >                                        kmem_cache_alloc(...)
> >                                        kmem_cache_alloc(...)
> >                                        kmem_cache_alloc(...)
> >                                        kmem_cache_alloc(...)
> >                                        ...
> >                                           eats it all
> > kmem_cache_alloc(...) -> ENOMEM
> >    who ate my donuts?!
> 
> Yes, that's my understanding also.
> 
> I can see why it's a problem in theory, but I don't think Peter has yet
> revealed to us why it's a problem in practice.  I got all excited when
> Christoph asked "I am not sure what the point of all of this is.", but
> Peter cunningly avoided answering that ;)
> 
> What observed problem is being fixed here?

I'm moving towards swapping over networked storage. Admittedly a new
feature.

Like with pretty much all other swap solutions; there is the fundamental
vm deadlock: freeing memory requires memory. Current block devices get
around that by using mempools. This works well.

However with network traffic mempools are not easily usable; the network
stack uses kmalloc. By using reserve based allocation we can keep
operating in a similar matter.