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Accepted
Jonathan Lynch
Asked: 2015-10-08 07:44:48 +0800 CST

free reports bogus amount of swap used

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The swap used reported by free is very high.

[root@rhel6 ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          9892       9537        354          0         71        884
-/+ buffers/cache:       8581       1310 
Swap:          767 1759218592     116869

Like, really high.

[root@bb14 blackboard]# free -g|grep Swap
Swap:            0 1717986906        114

Or is it?

[root@bb14 blackboard]# free -h |grep Swap
Swap:         767M       767M       114G

Even more strange, even if I disable swap the number still stays high.

[root@rhel6 ~]# swapoff -a
[root@rhel6 ~]# free -m
             total       used       free     shared    buffers     cached
Mem:          9892       9760        131          0         45        638
-/+ buffers/cache:       9076        815 
Swap:            0 1759218592     116814

Things don't get any less confusing when checking meminfo, which shows swapfree higher than swaptotal.

[root@rhel6 ~]# cat /proc/meminfo|grep Swap
SwapCached:            0 kB
SwapTotal:        786428 kB
SwapFree:       120404008 kB

Obviously something is wonky and my first instinct is to reboot, but this is a production machine which means maintenance windows, etc, and I'm wondering if there's any way to find out what's wrong and possibly even fix it without downtime.

linux

2 Answers

  1. Best Answer

    The solution was to Upgrade to kernel-2.6.32-573.7.1.el6 or higher.

    A simple yum update and reboot should be all it takes.

    Below is a quote of the bug report of kernel-2.6.32-573.1.1.el6.x86_64 that caused the Swap free to be greater than the swap total:

    A previous change in the get_swap_page() locking removed the use of the swap_lock spinlock. This could cause nr_swap_pages corruption and invalid SwapFree information in the /proc/meminfo file, where the size of SwapFree could exceed the size of SwapTotal. This update uses an atomic variable for nr_swap_pages, and the size of SwapFree in /proc/meminfo is now correct. (BZ#1259362)

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  2. It looks like this is a known issue in RHEL6.7 with kernel-2.6.32-573.1.1.el6.x86_64 being tracked in their private bugzilla.

    https://access.redhat.com/solutions/1571043

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