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Subject[PATCH 00/14] Swap-over-NBD without deadlocking v4r3
Date
Changelog since V3
o Propogate pfmemalloc from packet fragment pages to skb (Neil)
o Rebase to 3.0-rc2

Changelog since V2
o Document that __GFP_NOMEMALLOC overrides __GFP_MEMALLOC (Neil)
o Use wait_event_interruptible (Neil)
o Use !! when casting to bool to avoid any possibilitity of type
truncation (Neil)
o Nicer logic when using skb_pfmemalloc_protocol (Neil)

Changelog since V1
o Rebase on top of mmotm
o Use atomic_t for memalloc_socks (David Miller)
o Remove use of sk_memalloc_socks in vmscan (Neil Brown)
o Check throttle within prepare_to_wait (Neil Brown)
o Add statistics on throttling instead of printk

Swapping over NBD is something that is technically possible but not
often advised. While there are number of guides on the internet
on how to configure it and nbd-client supports a -swap switch to
"prevent deadlocks", the fact of the matter is a machine using NBD
for swap can be locked up within minutes if swap is used intensively.

The problem is that network block devices do not use mempools like
normal block devices do. As the host cannot control where they receive
packets from, they cannot reliably work out in advance how much memory
they might need.

Some years ago, Peter Ziljstra developed a series of patches that
supported swap over an NFS that some distributions are carrying in
their kernels. This patch series borrows very heavily from Peter's work
to support swapping over NBD (the relatively straight-forward case)
and uses throttling instead of dynamically resized memory reserves
so the series is not too unwieldy for review.

Patch 1 serialises access to min_free_kbytes. It's not strictly needed
by this series but as the series cares about watermarks in
general, it's a harmless fix. It could be merged independently.

Patch 2 adds knowledge of the PFMEMALLOC reserves to SLAB and SLUB to
preserve access to pages allocated under low memory situations
to callers that are freeying memory.

Patch 3 introduces __GFP_MEMALLOC to allow access to the PFMEMALLOC
reserves without setting PFMEMALLOC.

Patch 4 opens the possibility for softirqs to use PFMEMALLOC reserves
for later use by network packet processing.

Patch 5 ignores memory policies when ALLOC_NO_WATERMARKS is set.

Patches 6-10 allows network processing to use PFMEMALLOC reserves when
the socket has been marked as being used by the VM to clean
pages. If packets are received and stored in pages that were
allocated under low-memory situations and are unrelated to
the VM, the packets are dropped.

Patch 11 is a micro-optimisation to avoid a function call in the
common case.

Patch 12 tags NBD sockets as being SOCK_MEMALLOC so they can use
PFMEMALLOC if necessary.

Patch 13 notes that it is still possible for the PFMEMALLOC reserve
to be depleted. To prevent this, direct reclaimers get
throttled on a waitqueue if 50% of the PFMEMALLOC reserves are
depleted. It is expected that kswapd and the direct reclaimers
already running will clean enough pages for the low watermark
to be reached and the throttled processes are woken up.

Patch 14 adds a statistic to track how often processes get throttled

Some basic performance testing was run using kernel builds, netperf
on loopback for UDP and TCP, hackbench (pipes and sockets), iozone
and sysbench. Each of them were expected to use the sl*b allocators
reasonably heavily but there did not appear to be significant
performance variances. Here is the results from netperf using
slab as an example

NETPERF UDP
64 237.47 ( 0.00%) 237.34 (-0.05%)
128 472.69 ( 0.00%) 465.96 (-1.44%)
256 926.82 ( 0.00%) 948.40 ( 2.28%)
1024 3260.08 ( 0.00%) 3266.50 ( 0.20%)
2048 5535.11 ( 0.00%) 5453.55 (-1.50%)
3312 7496.60 ( 0.00%)* 7574.44 ( 1.03%)
1.12% 1.00%
4096 8266.35 ( 0.00%)* 8240.06 (-0.32%)*
1.18% 1.49%
8192 11026.01 ( 0.00%) 11010.44 (-0.14%)
16384 14653.98 ( 0.00%) 14666.97 ( 0.09%)
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 2156.64 1873.27
Total Elapsed Time (seconds) 2570.09 2234.10

NETPERF TCP
netperf-tcp tcp-swapnbd
vanilla-slab v4r3-slab
64 1250.76 ( 0.00%) 1256.52 ( 0.46%)
128 2290.70 ( 0.00%) 2336.43 ( 1.96%)
256 3668.42 ( 0.00%) 3751.17 ( 2.21%)
1024 7214.33 ( 0.00%) 7237.23 ( 0.32%)
2048 8230.01 ( 0.00%) 8280.02 ( 0.60%)
3312 8634.95 ( 0.00%) 8758.62 ( 1.41%)
4096 8851.18 ( 0.00%) 9045.88 ( 2.15%)
8192 10067.59 ( 0.00%) 10263.30 ( 1.91%)
16384 11523.26 ( 0.00%) 11654.78 ( 1.13%)
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 1450.23 1389.8
Total Elapsed Time (seconds) 1450.41 1390.35

Here is the equivalent test for SLUB

netperf-udp udp-swapnbd
vanilla-slub v4r3-slub
64 235.33 ( 0.00%) 237.80 ( 1.04%)
128 465.92 ( 0.00%) 469.98 ( 0.86%)
256 907.16 ( 0.00%) 907.58 ( 0.05%)
1024 3240.25 ( 0.00%) 3255.56 ( 0.47%)
2048 5564.87 ( 0.00%) 5446.46 (-2.17%)
3312 7427.65 ( 0.00%)* 7650.00 ( 2.91%)
1.33% 1.00%
4096 8004.51 ( 0.00%)* 8132.79 ( 1.58%)*
1.05% 1.21%
8192 11079.60 ( 0.00%) 10927.09 (-1.40%)
16384 14737.38 ( 0.00%) 15019.50 ( 1.88%)
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 2056.21 2160.38
Total Elapsed Time (seconds) 2426.09 2498.16

NETPERF TCP
netperf-tcp tcp-swapnbd
vanilla-slub v4r3-slub
64 1251.64 ( 0.00%) 1262.89 ( 0.89%)
128 2289.88 ( 0.00%) 2332.94 ( 1.85%)
256 3654.34 ( 0.00%) 3736.48 ( 2.20%)
1024 7192.47 ( 0.00%) 7286.96 ( 1.30%)
2048 8243.55 ( 0.00%) 8291.50 ( 0.58%)
3312 8664.16 ( 0.00%) 8799.88 ( 1.54%)
4096 8869.13 ( 0.00%) 9018.12 ( 1.65%)
8192 10009.53 ( 0.00%) 10214.26 ( 2.00%)
16384 11470.78 ( 0.00%) 11685.20 ( 1.83%)
MMTests Statistics: duration
User/Sys Time Running Test (seconds) 1368.28 1511.81
Total Elapsed Time (seconds) 1370.33 1510.42

Time to completion varied a lot but this can happen with netperf as
it tries to find results within a sufficiently high confidence. There
were some small gains and losses but they are close to the variances
seen between kernel releases.

For testing swap-over-NBD, a machine was booted with 2G of RAM with a
swapfile backed by NBD. 8*NUM_CPU processes were started that create
anonymous memory mappings and read them linearly in a loop. The total
size of the mappings were 4*PHYSICAL_MEMORY to use swap heavily under
memory pressure. Without the patches, the machine locks up within
minutes and runs to completion with them applied.

drivers/block/nbd.c | 7 +-
include/linux/gfp.h | 13 ++-
include/linux/mm_types.h | 8 ++
include/linux/mmzone.h | 1 +
include/linux/sched.h | 7 +
include/linux/skbuff.h | 21 +++-
include/linux/slub_def.h | 1 +
include/linux/vm_event_item.h | 1 +
include/net/sock.h | 19 +++
include/trace/events/gfpflags.h | 1 +
kernel/softirq.c | 3 +
mm/page_alloc.c | 57 +++++++--
mm/slab.c | 240 +++++++++++++++++++++++++++++++++------
mm/slub.c | 35 +++++-
mm/vmscan.c | 55 +++++++++
mm/vmstat.c | 1 +
net/core/dev.c | 48 +++++++-
net/core/filter.c | 8 ++
net/core/skbuff.c | 95 +++++++++++++---
net/core/sock.c | 42 +++++++
net/ipv4/tcp.c | 3 +-
net/ipv4/tcp_output.c | 13 +-
net/ipv6/tcp_ipv6.c | 12 ++-
23 files changed, 604 insertions(+), 87 deletions(-)
--
1.7.3.4

Mel Gorman (14):
mm: Serialize access to min_free_kbytes
mm: sl[au]b: Add knowledge of PFMEMALLOC reserve pages
mm: Introduce __GFP_MEMALLOC to allow access to emergency reserves
mm: allow PF_MEMALLOC from softirq context
mm: Ignore mempolicies when using ALLOC_NO_WATERMARK
net: Introduce sk_allocation() to allow addition of GFP flags
depending on the individual socket
netvm: Allow the use of __GFP_MEMALLOC by specific sockets
netvm: Allow skb allocation to use PFMEMALLOC reserves
netvm: Propagate page->pfmemalloc to skb
netvm: Set PF_MEMALLOC as appropriate during SKB processing
mm: Micro-optimise slab to avoid a function call
nbd: Set SOCK_MEMALLOC for access to PFMEMALLOC reserves
mm: Throttle direct reclaimers if PF_MEMALLOC reserves are low and
swap is backed by network storage
mm: Account for the number of times direct reclaimers get throttled

drivers/block/nbd.c | 7 +-
include/linux/gfp.h | 13 ++-
include/linux/mm_types.h | 8 ++
include/linux/mmzone.h | 1 +
include/linux/sched.h | 7 +
include/linux/skbuff.h | 21 +++-
include/linux/slub_def.h | 1 +
include/linux/vm_event_item.h | 1 +
include/net/sock.h | 19 +++
include/trace/events/gfpflags.h | 1 +
kernel/softirq.c | 3 +
mm/page_alloc.c | 57 +++++++--
mm/slab.c | 240 +++++++++++++++++++++++++++++++++------
mm/slub.c | 33 +++++-
mm/vmscan.c | 55 +++++++++
mm/vmstat.c | 1 +
net/core/dev.c | 48 +++++++-
net/core/filter.c | 8 ++
net/core/skbuff.c | 95 +++++++++++++---
net/core/sock.c | 42 +++++++
net/ipv4/tcp.c | 3 +-
net/ipv4/tcp_output.c | 13 +-
net/ipv6/tcp_ipv6.c | 12 ++-
23 files changed, 602 insertions(+), 87 deletions(-)
--
1.7.3.4



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