操作系统：CentOS 7.0

strace df的输出：

execve("/usr/bin/df",["df"],[/* 22 vars */]) = 0
brk(NULL)                               = 0x1499000
mmap(NULL,4096,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANONYMOUS,-1,0) = 0x7f628668f000
access("/etc/ld.so.preload",R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache",O_RDONLY|O_CLOEXEC) = 3
fstat(3,{st_mode=S_IFREG|0644,st_size=45250,...}) = 0
mmap(NULL,45250,PROT_READ,MAP_PRIVATE,3,0) = 0x7f6286683000
close(3)                                = 0
open("/lib64/libc.so.6",O_RDONLY|O_CLOEXEC) = 3
read(3,"\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P%\2\0\0\0\0\0"...,832) = 832
fstat(3,{st_mode=S_IFREG|0755,st_size=2173512,3981792,PROT_READ|PROT_EXEC,MAP_PRIVATE|MAP_DENYWRITE,0) = 0x7f62860a2000
mprotect(0x7f6286265000,2093056,PROT_NONE) = 0
mmap(0x7f6286464000,24576,MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE,0x1c2000) = 0x7f6286464000
mmap(0x7f628646a000,16864,MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS,0) = 0x7f628646a000
close(3)                                = 0
mmap(NULL,0) = 0x7f6286682000
mmap(NULL,8192,0) = 0x7f6286680000
arch_prctl(ARCH_SET_FS,0x7f6286680740) = 0
mprotect(0x7f6286464000,16384,PROT_READ) = 0
mprotect(0x616000,PROT_READ)     = 0
mprotect(0x7f6286690000,PROT_READ) = 0
munmap(0x7f6286683000,45250)           = 0
brk(NULL)                               = 0x1499000
brk(0x14ba000)                          = 0x14ba000
brk(NULL)                               = 0x14ba000
open("/usr/lib/locale/locale-archive",st_size=106070960,106070960,0) = 0x7f627fb79000
close(3)                                = 0
open("/usr/share/locale/locale.alias",st_size=2502,0) = 0x7f628668e000
read(3,"# Locale name alias data base.\n#"...,4096) = 2502
read(3,"",4096)                       = 0
close(3)                                = 0
munmap(0x7f628668e000,4096)            = 0
open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/coreutils.mo",O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US.utf8/LC_MESSAGES/coreutils.mo",O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US/LC_MESSAGES/coreutils.mo",O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.UTF-8/LC_MESSAGES/coreutils.mo",O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.utf8/LC_MESSAGES/coreutils.mo",O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/coreutils.mo",O_RDONLY) = -1 ENOENT (No such file or directory)
open("/etc/mtab",O_RDONLY|O_CLOEXEC)   = 3
fstat(3,{st_mode=S_IFREG|0444,st_size=0,"rootfs / rootfs rw 0 0\nsysfs /sy"...,1024) = 1024
read(3,"oexec,relatime,memory 0 0\ncgroup"...,"rw,nosuid,nodev,size=14"...,1024) = 167
read(3,1024)                       = 0
close(3)                                = 0
munmap(0x7f628668e000,4096)            = 0
stat("/",{st_mode=S_IFDIR|0555,st_size=4096,...}) = 0
stat("/sys",...}) = 0
stat("/proc",...}) = 0
stat("/dev",{st_mode=S_IFDIR|0755,st_size=2780,...}) = 0
stat("/sys/kernel/security",...}) = 0
stat("/dev/shm",{st_mode=S_IFDIR|S_ISVTX|0777,st_size=40,...}) = 0
stat("/dev/pts",...}) = 0
stat("/run",st_size=1100,...}) = 0
stat("/sys/fs/cgroup",st_size=340,...}) = 0
stat("/sys/fs/cgroup/systemd",...}) = 0
stat("/sys/fs/pstore",...}) = 0
stat("/sys/fs/cgroup/devices",...}) = 0
stat("/sys/fs/cgroup/pids",...}) = 0
stat("/sys/fs/cgroup/cpu,cpuacct",...}) = 0
stat("/sys/fs/cgroup/memory",...}) = 0
stat("/sys/fs/cgroup/hugetlb",...}) = 0
stat("/sys/fs/cgroup/freezer",...}) = 0
stat("/sys/fs/cgroup/blkio",...}) = 0
stat("/sys/fs/cgroup/perf_event",...}) = 0
stat("/sys/fs/cgroup/cpuset",...}) = 0
stat("/sys/fs/cgroup/net_cls,net_prio",...}) = 0
stat("/sys/kernel/config",...}) = 0
stat("/",...}) = 0
stat("/proc/sys/fs/binfmt_misc",

fstab文件：

/dev/xvda1              /                       ext4     defaults,noatime 1 1
/swapfile   swap    swap    sw  0   0