[PATCH 00/28] Swap over NFS -v16

Peter Zijlstra
2008-02-20T15:23:49Z

Hi,

Another posting of the full swap over NFS series.

Andrew/Linus, could we start thinking of sticking this in -mm?

[ patches against 2.6.25-rc2-mm1, also to be found online at:
http://programming.kicks-ass.net/kernel-patches/vm_deadlock/v2.6.25-rc2-mm1/ ]

The patch-set can be split in roughtly 5 parts, for each of which I shall give
a description.

Part 1, patches 1-11

The problem with swap over network is the generic swap problem: needing memory
to free memory. Normally this is solved using mempools, as can be seen in the
BIO layer.

Swap over network has the problem that the network subsystem does not use fixed
sized allocations, but heavily relies on kmalloc(). This makes mempools
unusable.

This first part provides a generic reserve framework. This framework
could also be used to get rid of some of the __GFP_NOFAIL users.

Care is taken to only affect the slow paths - when we're low on memory.

Caveats: it currently doesn't do SLOB.

1 - mm: gfp_to_alloc_flags()
2 - mm: tag reseve pages
3 - mm: sl[au]b: add knowledge of reserve pages
4 - mm: kmem_estimate_pages()
5 - mm: allow PF_MEMALLOC from softirq context
6 - mm: serialize access to min_free_kbytes
7 - mm: emergency pool
8 - mm: system wide ALLOC_NO_WATERMARK
9 - mm: __GFP_MEMALLOC
10 - mm: memory reserve management
11 - selinux: tag avc cache alloc as non-critical

Part 2, patches 12-14

Provide some generic network infrastructure needed later on.

12 - net: wrap sk->sk_backlog_rcv()
13 - net: packet split receive api
14 - net: sk_allocation() - concentrate socket related allocations

Part 3, patches 15-21

Now that we have a generic memory reserve system, use it on the network stack.
The thing that makes this interesting is that, contrary to BIO, both the
transmit and receive path require memory allocations.

That is, in the BIO layer write back completion is usually just an ISR flipping
a bit and waking stuff up. A network write back completion involved receiving
packets, which when there is no memory, is rather hard. And even when there is
memory there is no guarantee that the required packet comes in in the window
that that memory buys us.

The solution to this problem is found in the fact that network is to be assumed
lossy. Even now, when there is no memory to receive packets the network card
will have to discard packets. What we do is move this into the network stack.

So we reserve a little pool to act as a receive buffer, this allows us to
inspect packets before tossing them. This way, we can filter out those packets
that ensure progress (writeback completion) and disregard the others (as would
have happened anyway). [ NOTE: this is a stable mode of operation with limited
memory usage, exactly the kind of thing we need ]

Again, care is taken to keep much of the overhead of this to only affect the
slow path. Only packets allocated from the reserves will suffer the extra
atomic overhead needed for accounting.

15 - netvm: network reserve infrastructure
16 - netvm: INET reserves.
17 - netvm: hook skb allocation to reserves
18 - netvm: filter emergency skbs.
19 - netvm: prevent a TCP specific deadlock
20 - netfilter: NF_QUEUE vs emergency skbs
21 - netvm: skb processing

Part 4, patches 22-23

Generic vm infrastructure to handle swapping to a filesystem instead of a block
device.

This provides new a_ops to handle swapcache pages and could be used to obsolete
the bmap usage for swapfiles.

22 - mm: add support for non block device backed swap files
23 - mm: methods for teaching filesystems about PG_swapcache pages

Part 5, patches 24-28

Finally, convert NFS to make use of the new network and vm infrastructure to
provide swap over NFS.

24 - nfs: remove mempools
25 - nfs: teach the NFS client how to treat PG_swapcache pages
26 - nfs: disable data cache revalidation for swapfiles
27 - nfs: enable swap on NFS
28 - nfs: fix various memory recursions possible with swap over NFS.

Changes since -v15:
- fwd port
- more SGE fragment drivers ported
- made the new swapfile logic unconditional
- various bug fixes and cleanups