Re: [PATCH] Deadlock during heavy write activity to userspace NFSserver on local NFS mount

[Avi Kivity]

Pavel Machek wrote:

> > On heavy write activity, allocators wait synchronously for kswapd to
> > free some memory. But if kswapd is freeing memory via a userspace NFS
> > server, that server could be waiting for kswapd, and the system seizes
> > instantly.
> >
> > This patch (against RHEL 2.4.21-15EL, but should apply either 
> > literally
> > or conceptually to other kernels) allows a process to declare itself 
> > as
> > kswapd's little helper, and thus will not have to wait on kswapd.
> >    
>
> Ok, but what if its memory runs out, anyway?
>
>  
Tough. What if kswapd's memory runs out?

A more complete solution would be to assign memory reserve levels below 
which a process starts allocating synchronously. For example, normal 
processes must have >20MB to make forward progress, kswapd wants >15MB 
and the NFS server needs >10MB. Some way would be needed to express the 
dependencies.

I think more and more people will hit this problem as filesystems become 
more complex due to clustering, and migrate to userspace where it can be 
more easily managed.

Avi
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