wait until swapoff is *actually* finished (it returns too early)?

[Thorsten Glaser <tg@debian.org>]

(Please d̲o̲ Cc me on replies, I don’t subscribe to this list. Thanks!)

Hi,

this is a bit curious problem:

I have a setup with swap devices on dmcrypt:

$ cat /etc/crypttab
# <target name> <source device>         <key file>      <options>
crtpv           LABEL=fooclvm           none            discard,luks,initramfs
cswp1           /dev/vg-foo/lv-swp1     /dev/random     discard,cipher=aes-xts-plain64,size=256,plain,swap
cswp2           /dev/vg-foo/lv-swp2     /dev/random     discard,cipher=aes-xts-plain64,size=256,plain,swap

In a cronjob, I basically do swapoff && cryptdisks_stop && \
cryptdisks_start && swapon for both swaps individually to
throw away the old encryption key regularily (but not too
frequently).

I immediately ran into the problem, when trying this for the
first time, that a “swapoff /dev/mapper/cswp1” returns before
the device is released, so the subsequent cryptdisks_stop fails.

I found that inserting a “cat /proc/swaps”, funnily enough,
makes those failures less frequent but still present; adding
a “sleep 3” as well made it work for months.

Until tonight when it didn’t.

Just adding a “sleep” is no proper fix anyway, so the question
is, how to wait in a shell script until the swap device is
*really* swapoff’d when the syscall returns too early, and
(someone from the Linux kernel maintainers reading this?) should
I report the latter as a bug against the kernel?

This is on bullseye/amd64, on VMs and bare metal both. Using
direct partitions like /dev/sda3 (or via LABEL= to avoid trouble)
makes no difference from using LVs.

Thanks in advance,
//mirabilos
-- 
16:47⎜«mika:#grml» .oO(mira ist einfach gut....)      23:22⎜«mikap:#grml»
mirabilos: und dein bootloader ist geil :)    23:29⎜«mikap:#grml» und ich
finds saugeil dass ich ein bsd zum booten mit grml hab, das muss ich dann
gleich mal auf usb-stick installieren	-- Michael Prokop über MirOS bsd4grml