Linux – oom-killer killing processes despite having plenty of free swap

This machine has a ton of swap, yet processes still occasionally get killed by the oom-killer. Can anyone explain this behavior, and more importantly how to keep it from occurring?

Dmesg output:

python invoked oom-killer: gfp_mask=0x1200d2, order=0, oomkilladj=4
Pid: 13996, comm: python Not tainted 2.6.27-gentoo-r8cluster-e1000 #9

Call Trace:
 [<ffffffff8025ab6b>] oom_kill_process+0x57/0x1dc
 [<ffffffff802460c7>] getnstimeofday+0x53/0xb3
 [<ffffffff8025ae78>] badness+0x16a/0x1a9
 [<ffffffff8025b0a9>] out_of_memory+0x1f2/0x25c
 [<ffffffff8025e181>] __alloc_pages_internal+0x30f/0x3b2
 [<ffffffff8026fea0>] read_swap_cache_async+0x48/0xc0
 [<ffffffff8026ff6f>] swapin_readahead+0x57/0x98
 [<ffffffff80266d0e>] handle_mm_fault+0x408/0x706
 [<ffffffff8057da33>] do_page_fault+0x42c/0x7e7
 [<ffffffff8057baf9>] error_exit+0x0/0x51

Mem-Info:
Node 0 DMA per-cpu:
CPU    0: hi:    0, btch:   1 usd:   0
CPU    1: hi:    0, btch:   1 usd:   0
CPU    2: hi:    0, btch:   1 usd:   0
CPU    3: hi:    0, btch:   1 usd:   0
Node 0 DMA32 per-cpu:
CPU    0: hi:  186, btch:  31 usd: 103
CPU    1: hi:  186, btch:  31 usd:  48
CPU    2: hi:  186, btch:  31 usd: 136
CPU    3: hi:  186, btch:  31 usd: 183
Active:480346 inactive:483 dirty:0 writeback:10 unstable:0
 free:3408 slab:5146 mapped:1408 pagetables:2687 bounce:0
Node 0 DMA free:8024kB min:20kB low:24kB high:28kB active:1156kB inactive:0kB present:8364kB pages_scanned:3246 all_unreclaimable? yes
lowmem_reserve[]: 0 2003 2003 2003
Node 0 DMA32 free:5608kB min:5716kB low:7144kB high:8572kB active:1920228kB inactive:1932kB present:2051308kB pages_scanned:2941301 all_unreclaimable? yes
lowmem_reserve[]: 0 0 0 0
Node 0 DMA: 8*4kB 3*8kB 4*16kB 3*32kB 4*64kB 3*128kB 2*256kB 3*512kB 3*1024kB 1*2048kB 0*4096kB = 8024kB
Node 0 DMA32: 42*4kB 6*8kB 1*16kB 0*32kB 2*64kB 1*128kB 0*256kB 0*512kB 1*1024kB 0*2048kB 1*4096kB = 5608kB
325424 total pagecache pages
323900 pages in swap cache
Swap cache stats: add 20776604, delete 20452704, find 7856195/10744535
Free swap  = 151691424kB
Total swap = 156290896kB
524032 pages RAM
9003 pages reserved
331431 pages shared
186210 pages non-shared
Out of memory: kill process 12965 (bash) score 2236480 or a child
Killed process 13996 (python)

VM-related sysctls:

vm.overcommit_memory = 0
vm.panic_on_oom = 0
vm.oom_kill_allocating_task = 0
vm.oom_dump_tasks = 0
vm.overcommit_ratio = 50
vm.page-cluster = 3
vm.dirty_background_ratio = 5
vm.dirty_ratio = 10
vm.dirty_writeback_centisecs = 500
vm.dirty_expire_centisecs = 3000
vm.nr_pdflush_threads = 2
vm.swappiness = 60
vm.nr_hugepages = 0
vm.hugetlb_shm_group = 0
vm.hugepages_treat_as_movable = 0
vm.nr_overcommit_hugepages = 0
vm.lowmem_reserve_ratio = 256   256 32
vm.drop_caches = 0
vm.min_free_kbytes = 5740
vm.percpu_pagelist_fraction = 0
vm.max_map_count = 65536
vm.laptop_mode = 0
vm.block_dump = 0
vm.vfs_cache_pressure = 100
vm.legacy_va_layout = 0
vm.zone_reclaim_mode = 0
vm.min_unmapped_ratio = 1
vm.min_slab_ratio = 5
vm.stat_interval = 1
vm.numa_zonelist_order = default