Re: [RFC][PATCH 0/9] Network receive deadlock prevention for NBD

From: Evgeniy Polyakov
Date: Sat Aug 12 2006 - 10:47:50 EST

On Sat, Aug 12, 2006 at 10:40:23AM -0400, Rik van Riel (riel@xxxxxxxxxx) wrote:
> Evgeniy Polyakov wrote:
> >On Sat, Aug 12, 2006 at 11:19:49AM +0200, Peter Zijlstra
> >(a.p.zijlstra@xxxxxxxxx) wrote:
> >>>As you described above, memory for each packet must be allocated (either
> >>>from SLAB or from reserve), so network needs special allocator in OOM
> >>>condition, and that allocator should be separated from SLAB's one which
> >>>got OOM, so my purpose is just to use that different allocator (with
> >>>additional features) for netroking always.
> >
> >No it is not. There are socket queues and they are limited. Things like
> >TCP behave even better.
> Ahhh, but there are two allocators in play here.
> The first one allocates the memory for receiving packets.
> This can be one pool, as long as it is isolated from
> other things in the system it is fine.
> The second allocator allocates more memory for socket
> buffers. The memory critical sockets should get their
> memory from a mempool, once normal socket memory
> allocations start failing.
> This means our allocation differentiation only needs
> to happen at the socket stage.
> Or am I overlooking something?

Yep. Socket allocations end up with alloc_skb() which is essentialy the
same as what is being done for receiving path skbs.
If you really want to separate critical from non-critical sockets, it is
much better not to play with alloc_skb() but directly forbid it in
appropriate socket allocation function like sock_alloc_send_skb().

What I suggested in previous e-mail is to separate networking
allocations from other system allocations, so problem in main allocator
and it's OOM would never affect network path.

Evgeniy Polyakov
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