I removed all linux-backports-modules-* packages and the problem is still reproducible. Again, doing "cat /dev/sda > /dev/null " causes this only when the ath9k module is loaded.

Jul  1 07:43:59 cheren kernel: [  418.094914] __ratelimit: 6 callbacks suppressed
Jul  1 07:43:59 cheren kernel: [  418.094920] swapper: page allocation failure. order:1, mode:0x4020
Jul  1 07:43:59 cheren kernel: [  418.094927] Pid: 0, comm: swapper Not tainted 2.6.28-13-generic #44-Ubuntu
Jul  1 07:43:59 cheren kernel: [  418.094931] Call Trace:
Jul  1 07:43:59 cheren kernel: [  418.094943]  [<c04fca36>] ? printk+0x18/0x1a
Jul  1 07:43:59 cheren kernel: [  418.094951]  [<c0194287>] __alloc_pages_internal+0x387/0x490
Jul  1 07:43:59 cheren kernel: [  418.094959]  [<c01b7ebc>] ? __slab_alloc+0xcc/0x260
Jul  1 07:43:59 cheren kernel: [  418.094965]  [<c01943e1>] __get_free_pages+0x21/0x40
Jul  1 07:43:59 cheren kernel: [  418.094971]  [<c01b8c2e>] __kmalloc_track_caller+0xee/0xf0
Jul  1 07:43:59 cheren kernel: [  418.094979]  [<c0427eac>] ? __alloc_skb+0x2c/0x110
Jul  1 07:43:59 cheren kernel: [  418.094986]  [<c042818c>] ? dev_alloc_skb+0x1c/0x30
Jul  1 07:43:59 cheren kernel: [  418.094992]  [<c0427ecd>] __alloc_skb+0x4d/0x110
Jul  1 07:43:59 cheren kernel: [  418.094999]  [<c042818c>] dev_alloc_skb+0x1c/0x30
Jul  1 07:43:59 cheren kernel: [  418.095031]  [<f8019bc0>] ath_rxbuf_alloc+0x20/0x80 [ath9k]
Jul  1 07:43:59 cheren kernel: [  418.095053]  [<f8019c4c>] ath_rx_indicate+0x2c/0xd0 [ath9k]
Jul  1 07:43:59 cheren kernel: [  418.095074]  [<f801a01d>] ath_rx_tasklet+0x32d/0x520 [ath9k]
Jul  1 07:43:59 cheren kernel: [  418.095082]  [<c0119a73>] ? lapic_next_event+0x13/0x20
Jul  1 07:43:59 cheren kernel: [  418.095104]  [<f8022dee>] ath9k_tasklet+0x6e/0xa0 [ath9k]
Jul  1 07:43:59 cheren kernel: [  418.095112]  [<c013ed18>] tasklet_action+0x78/0x100
Jul  1 07:43:59 cheren kernel: [  418.095118]  [<c013f197>] __do_softirq+0x97/0x170
Jul  1 07:43:59 cheren kernel: [  418.095125]  [<c011c7df>] ? ack_apic_level+0x6f/0x290
Jul  1 07:43:59 cheren kernel: [  418.095132]  [<c013f2cd>] do_softirq+0x5d/0x60
Jul  1 07:43:59 cheren kernel: [  418.095138]  [<c013f445>] irq_exit+0x55/0x90
Jul  1 07:43:59 cheren kernel: [  418.095144]  [<c0106853>] do_IRQ+0x83/0xa0
Jul  1 07:43:59 cheren kernel: [  418.095150]  [<c01051f3>] common_interrupt+0x23/0x30
Jul  1 07:43:59 cheren kernel: [  418.095157]  [<c0319467>] ? acpi_idle_enter_bm+0x269/0x2b8
Jul  1 07:43:59 cheren kernel: [  418.095165]  [<c040b44f>] cpuidle_idle_call+0x6f/0xd0
Jul  1 07:43:59 cheren kernel: [  418.095170]  [<c010285d>] cpu_idle+0x6d/0xd0
Jul  1 07:43:59 cheren kernel: [  418.095177]  [<c04ed07e>] rest_init+0x4e/0x60
Jul  1 07:43:59 cheren kernel: [  418.095181] Mem-Info:
Jul  1 07:43:59 cheren kernel: [  418.095184] DMA per-cpu:
Jul  1 07:43:59 cheren kernel: [  418.095188] CPU    0: hi:    0, btch:   1 usd:   0
Jul  1 07:43:59 cheren kernel: [  418.095192] CPU    1: hi:    0, btch:   1 usd:   0
Jul  1 07:43:59 cheren kernel: [  418.095195] Normal per-cpu:
Jul  1 07:43:59 cheren kernel: [  418.095199] CPU    0: hi:  186, btch:  31 usd:  22
Jul  1 07:43:59 cheren kernel: [  418.095203] CPU    1: hi:  186, btch:  31 usd:  31
Jul  1 07:43:59 cheren kernel: [  418.095207] HighMem per-cpu:
Jul  1 07:43:59 cheren kernel: [  418.095210] CPU    0: hi:   42, btch:   7 usd:   0
Jul  1 07:43:59 cheren kernel: [  418.095214] CPU    1: hi:   42, btch:   7 usd:   7
Jul  1 07:43:59 cheren kernel: [  418.095222] Active_anon:33196 active_file:16912 inactive_anon:33295
Jul  1 07:43:59 cheren kernel: [  418.095224]  inactive_file:146758 unevictable:472 dirty:15 writeback:0 unstable:0
Jul  1 07:43:59 cheren kernel: [  418.095226]  free:5239 slab:6614 mapped:18554 pagetables:972 bounce:0
Jul  1 07:43:59 cheren kernel: [  418.095233] DMA free:3460kB min:64kB low:80kB high:96kB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:5084kB unevictable:0kB present:15764kB pages_scanned:0 all_unreclaimable? no
Jul  1 07:43:59 cheren kernel: [  418.095238] lowmem_reserve[]: 0 861 998 998
Jul  1 07:43:59 cheren kernel: [  418.095250] Normal free:7680kB min:3720kB low:4648kB high:5580kB active_anon:118236kB inactive_anon:118428kB active_file:29852kB inactive_file:525028kB unevictable:8kB present:881880kB pages_scanned:0 all_unreclaimable? no
Jul  1 07:43:59 cheren kernel: [  418.095256] lowmem_reserve[]: 0 0 1101 1101
Jul  1 07:43:59 cheren kernel: [  418.095268] HighMem free:9816kB min:136kB low:284kB high:432kB active_anon:14548kB inactive_anon:14752kB active_file:37796kB inactive_file:56920kB unevictable:1880kB present:141040kB pages_scanned:0 all_unreclaimable? no
Jul  1 07:43:59 cheren kernel: [  418.095274] lowmem_reserve[]: 0 0 0 0
Jul  1 07:43:59 cheren kernel: [  418.095282] DMA: 5*4kB 0*8kB 1*16kB 1*32kB 1*64kB 0*128kB 1*256kB 0*512kB 1*1024kB 1*2048kB 0*4096kB = 3460kB
Jul  1 07:43:59 cheren kernel: [  418.095301] Normal: 1746*4kB 1*8kB 0*16kB 0*32kB 0*64kB 1*128kB 0*256kB 1*512kB 0*1024kB 0*2048kB 0*4096kB = 7632kB
Jul  1 07:43:59 cheren kernel: [  418.095320] HighMem: 523*4kB 259*8kB 87*16kB 38*32kB 14*64kB 5*128kB 2*256kB 2*512kB 0*1024kB 0*2048kB 0*4096kB = 9844kB
Jul  1 07:43:59 cheren kernel: [  418.095339] 165614 total pagecache pages
Jul  1 07:43:59 cheren kernel: [  418.095343] 28 pages in swap cache
Jul  1 07:43:59 cheren kernel: [  418.095347] Swap cache stats: add 28, delete 0, find 0/0
Jul  1 07:43:59 cheren kernel: [  418.095350] Free swap  = 1390888kB
Jul  1 07:43:59 cheren kernel: [  418.095354] Total swap = 1391000kB
Jul  1 07:43:59 cheren kernel: [  418.098839] 261840 pages RAM
Jul  1 07:43:59 cheren kernel: [  418.098839] 35538 pages HighMem
Jul  1 07:43:59 cheren kernel: [  418.098839] 13659 pages reserved
Jul  1 07:43:59 cheren kernel: [  418.098839] 229512 pages shared
Jul  1 07:43:59 cheren kernel: [  418.098839] 96616 pages non-shared
Jul  1 07:43:59 cheren kernel: [  418.098839] ath_rxbuf_alloc: skbuff alloc of size 3904 failed
Jul  1 07:43:59 cheren kernel: [  418.402297] swapper: page allocation failure. order:1, mode:0x4020
Jul  1 07:43:59 cheren kernel: [  418.402304] Pid: 0, comm: swapper Not tainted 2.6.28-13-generic #44-Ubuntu
Jul  1 07:43:59 cheren kernel: [  418.402308] Call Trace:
etc....