Re: [PATCH] Repeated fork() causes SLAB to grow without bound

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

On Fri, 14 Nov 2014 10:30:53 -0600 Daniel Forrest <dan.forrest@ssec.wisc.edu> wrote:

> There have been a couple of inquiries about the status of this patch
> over the last few months, so I am going to try pushing it out.
> 
> Andrea Arcangeli has commented:
> 
> > Agreed. The only thing I don't like about this patch is the hardcoding
> > of number 5: could we make it a variable to tweak with sysfs/sysctl so
> > if some weird workload arises we have a tuning tweak? It'd cost one
> > cacheline during fork, so it doesn't look excessive overhead.
> 
> Adding this is beyond my experience level, so if it is required then
> someone else will have to make it so.
> 
> Rik van Riel has commented:
> 
> > I believe we should just merge that patch.
> > 
> > I have not seen any better ideas come by.
> > 
> > The comment should probably be fixed to reflect the
> > chain length of 5 though :)
> 
> So here is Michel's patch again with "(length > 1)" modified to
> "(length > 5)" and fixed comments.
> 
> I have been running with this patch (with the threshold set to 5) for
> over two years now and it does indeed solve the problem.
> 
> ---
> 
> anon_vma_clone() is modified to return the length of the existing
> same_vma anon vma chain, and we create a new anon_vma in the child
> if it is more than five forks after the anon_vma was created, as we
> don't want the same_vma chain to grow arbitrarily large.

hoo boy, what's going on here.

- Under what circumstances are we seeing this slab windup?

- What are the consequences?  Can it OOM the machine?

- Why is this occurring?  There aren't an infinite number of vmas, so
  there shouldn't be an infinite number of anon_vmas or
  anon_vma_chains.

- IOW, what has to be done to fix this properly?

- What are the runtime consequences of limiting the length of the chain?

> ...
>
> @@ -331,10 +334,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>  	 * First, attach the new VMA to the parent VMA's anon_vmas,
>  	 * so rmap can find non-COWed pages in child processes.
>  	 */
> -	if (anon_vma_clone(vma, pvma))
> +	length = anon_vma_clone(vma, pvma);
> +	if (length < 0)
>  		return -ENOMEM;

This should propagate the anon_vma_clone() return val instead of
assuming ENOMEM.  But that won't fix anything...

> +	else if (length > 5)
> +		return 0;
>  
> -	/* Then add our own anon_vma. */
> +	/*
> +	 * Then add our own anon_vma. We do this only for five forks after
> +	 * the anon_vma was created, as we don't want the same_vma chain to
> +	 * grow arbitrarily large.
> +	 */
>  	anon_vma = anon_vma_alloc();