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

[Avi Kivity]

Nick Piggin wrote:

> Avi Kivity wrote:
>
> > Nick Piggin wrote:
> >
> > > Avi Kivity wrote:
> > >
> > > > Nick Piggin wrote:
> > >
> > >
> > >
> > > > > > What's stopping the NFS server from ooming the machine then? 
> > > > > > Every time some bit of memory becomes free, the server will 
> > > > > > consume it instantly. Eventually ext3 will not be able to write 
> > > > > > anything out because it is out of memory.
> > > > > The NFS server should do the writeout a page at a time.
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > The NFS server writes not only in response to page reclaim (as a 
> > > > local NFS client), but also in response to pressure from non-local 
> > > > clients. If both ext3 and NFS have the same allocation limits, NFS 
> > > > may starve out ext3.
> > >
> > > What do you mean starve out ext3? ext3 gets written to *by the NFS 
> > > server*
> > > which is PF_MEMALLOC. 
> >
> >
> >
> > When the NFS server writes, it allocates pagecache and temporary 
> > objects. When ext3 writes, it allocates temporary objects. If the NFS 
> > server writes too much, ext3 can't allocate memory, and will never be 
> > able to allocate memory.
>
> That is because your NFS server shouldn't hog as much memory as
> it likes when it is PF_MEMALLOC. The entire writeout path should
> do a page at a time if it is PF_MEMALLOC. Ie, the server should
> be doing write, fsync. 

We attempted to use sync local mounts (not what you are suggesting: on 
the NFS client side, without the PF_MEMALLOC hack) and still got the 
same deadlock. I am unable to explain why.

> But now that I think about it, I guess you may not be able to
> distinguish that from regular writeout, so doing a page at a time
> would hurt performance too much.
>
> Hmm so I guess the idea of a per task reserve limit may be the way
> to do it, yes. Thanks for bearing with me!

It was my pleasure.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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