Understand this error

system logger process (PID 6517) was terminated by signal 9
background writer process (PID 6519) was terminated by signal 9
terminating any other active server processes

and OS in var/logs:

 kernel:  [<ffffffff800ba475>] out_of_memory+0x53/0x267
 kernel:  [<ffffffff8000f012>] __alloc_pages+0x229/0x2b2
 kernel:  [<ffffffff80031f4b>] read_swap_cache_async+0x45/0xd8
 kernel:  [<ffffffff800bf60c>] swapin_readahead+0x60/0xd3
 kernel:  [<ffffffff80008f3a>] __handle_mm_fault+0x952/0xdf2
 kernel:  [<ffffffff800127fd>] sock_def_readable+0x34/0x5f
 kernel:  [<ffffffff80251481>] unix_dgram_sendmsg+0x43d/0x4cf
 kernel:  [<ffffffff800645a5>] do_page_fault+0x4b8/0x81d
 kernel:  [<ffffffff80037264>] do_sock_write+0xc4/0xce
 kernel:  [<ffffffff8008630f>] dequeue_task+0x18/0x37
 kernel:  [<ffffffff80060ab8>] thread_return+0x0/0xea
 kernel:  [<ffffffff8005be1d>] error_exit+0x0/0x84
 kernel:  [<ffffffff8008bfb3>] do_syslog+0x173/0x3ae
 kernel:  [<ffffffff8008bf81>] do_syslog+0x141/0x3ae
 kernel:  [<ffffffff8009b666>] autoremove_wake_function+0x0/0x2e
 kernel:  [<ffffffff800f65fd>] kmsg_read+0x3a/0x44
 kernel:  [<ffffffff8000b212>] vfs_read+0xcb/0x171
 kernel:  [<ffffffff8001145c>] sys_read+0x45/0x6e
 kernel:  [<ffffffff8005b2c1>] tracesys+0xd1/0xdc

 kernel: Free swap  = 0kB
 kernel: Total swap = 2031608kB
 kernel: Free swap:            0kB
kernel: 4390912 pages of RAM
 kernel: 280785 reserved pages
 kernel: 10222 pages shared
kernel: 4 pages swap cached
kernel: Out of memory: Killed process 6519 (postmaster).

How prenvent postgres use all memory of system?Why this happen?

Thanks for all
Paulo