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I've recently noticed that even though there's a lot of memory on my system:

# free -m
             total       used       free     shared    buffers     cached
Mem:         15965       9680       6284         35       1754       2560
-/+ buffers/cache:       5365      10599
Swap:         2047        258       1789

I cannot swapoff -a with my 2G swapfile

-rw-r----- 1 root root 2.0G Feb  9 21:34 /2GB.swap

I've changed the following kernel parameters via sysctl but that should be the cause:

vm.swappiness = 5
vm.vfs_cache_pressure = 200

Any reason for:

# swapoff -a
swapoff: /2GB.swap: swapoff failed: Cannot allocate memory

? dmesg doesn't log anything when swapoff fails. I'm using linux kernel 4.19.20-041920-generic.


Another example including /proc/meminfo

# cat /proc/meminfo
MemTotal:       16348296 kB
MemFree:         6673788 kB
MemAvailable:   11233052 kB
Buffers:          525048 kB
Cached:          2837788 kB
SwapCached:       362556 kB
Active:          4728244 kB
Inactive:        2758260 kB
Active(anon):    3132940 kB
Inactive(anon):  1043676 kB
Active(file):    1595304 kB
Inactive(file):  1714584 kB
Unevictable:        2396 kB
Mlocked:            2396 kB
SwapTotal:       2097148 kB
SwapFree:        1124272 kB
Dirty:               336 kB
Writeback:             0 kB
AnonPages:       3786868 kB
Mapped:           699944 kB
Shmem:             53116 kB
Slab:            1770268 kB
SReclaimable:    1578564 kB
SUnreclaim:       191704 kB
KernelStack:       47216 kB
PageTables:        82968 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    10271296 kB
Committed_AS:   24712604 kB
VmallocTotal:   34359738367 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB
Percpu:             3552 kB
HardwareCorrupted:     0 kB
AnonHugePages:     26624 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
Hugetlb:               0 kB
DirectMap4k:      714480 kB
DirectMap2M:    13891584 kB
DirectMap1G:     3145728 kB

and free -m

# free -m
             total       used       free     shared    buffers     cached
Mem:         15965       9447       6517         51        512       2771
-/+ buffers/cache:       6163       9801
Swap:         2047        950       1097

EDIT

strace swapoff -a

root@MACHINE:~# strace swapoff -a
execve("/sbin/swapoff", ["swapoff", "-a"], [/* 22 vars */]) = 0
...
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=9469488, ...}) = 0
mmap(NULL, 9469488, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd262737000
close(3)                                = 0
open("/proc/swaps", O_RDONLY)           = 3
fstat(3, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd263a54000
read(3, "Filename\t\t\t\tType\t\tSize\tUsed\tPrio"..., 1024) = 102
readlink("/2GB.swap", 0x7ffcfbb3bea0, 4096) = -1 EINVAL (Invalid argument)
read(3, "", 1024)                       = 0
close(3)                                = 0
munmap(0x7fd263a54000, 4096)            = 0
swapoff("/2GB.swap")                    = -1 ENOMEM (Cannot allocate memory)
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2570, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd263a54000
read(3, "# Locale name alias data base.\n#"..., 4096) = 2570
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fd263a54000, 4096)            = 0
open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/util-linux.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
...
open("/usr/share/locale-langpack/en/LC_MESSAGES/util-linux.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
write(2, "swapoff: ", 9swapoff: )                = 9
write(2, "/2GB.swap: swapoff failed", 25/2GB.swap: swapoff failed) = 25
write(2, ": ", 2: )                       = 2
open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
...
open("/usr/share/locale-langpack/en/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
write(2, "Cannot allocate memory\n", 23Cannot allocate memory
) = 23
open("/etc/fstab", O_RDONLY|O_CLOEXEC)  = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=838, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd263a54000
read(3, "# /etc/fstab: static file system"..., 4096) = 838
readlink("/2GB.swap", 0x7ffcfbb3c2b0, 4096) = -1 EINVAL (Invalid argument)
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fd263a54000, 4096)            = 0
exit_group(-1)                          = ?
+++ exited with 255 +++
  • If your system is swapping, removing swap partition will not solve any problem. You will just create additional problems (and BTW it is better to use a partition, then a file). You may need to find a instant where programs are not swapping data into that file. Maybe you should STOP such programs. Note: fragmented memory could be a reason. Try maybe also few time with sync to reduce dirty pages. – Giacomo Catenazzi Feb 10 at 10:49
  • 1
    @GiacomoCatenazzi 1) Why is it better to use a partition than a file? 2) sync doesn't change memory utilisation one bit – Patryk Feb 10 at 10:55
  • 1
    sync will write dirty pages to disk (not all, but when these pages should be on disk). This makes such pages clean, so they can be discarded easily (without swapping, or writing to disk), so kernel can quickly transform part of memory from cached to free. It is a very short term trick, to shorten critical phases (swapoff/umount). – Giacomo Catenazzi Feb 10 at 13:04
  • Partition is in direct control of kernel (single contiguous block, aligned). Files: there is risk not to have enough contiguous space or interference from other system utilities). [and ideal root filesystem should be read-only]. [And often it is useful to have a temporary partition, to recover system, or to so some "invasive" system handling, especially on remote machines, or machines without easy bootable device]. Not an hard requirements, but I found very often less problematic to have a special partition for that [and for RAID machines you can unRAID swap, for speed] – Giacomo Catenazzi Feb 10 at 13:13
  • 1
    The issue is still... There is plenty of free RAM, more than the TOTAL size of the swap file. Why can't swap be disabled? – Paul Stelian Feb 11 at 9:38

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