"cannot set up guest memory" b/c no automatic clearing of Linux' cache

Version: qemu-2.6.0-1
Kernel: 4.4.13-1-MANJARO
Full script (shouldn't matter though): https://pastebin.com/Hp24PWNE

Problem:
When host has been up and used for a while cache has been filled as much that guest can't be started without droping caches.

Expected behavior:
Qemu should be able to request as much Memory as it needs and cause Linux to drop cache pages if needed. A user shouldn't be required to have to come to this conclusion and having to drop caches to start Qemu with the required amount of memory.

My fix:
Following command (as root) required before qemu start:
# sync && echo 3 > /proc/sys/vm/drop_caches

Example:
$ sudo qemu.sh -m 10240 && echo success || echo failed
qemu-system-x86_64: cannot set up guest memory 'pc.ram': Cannot allocate memory
failed
$ free
              total        used        free      shared  buff/cache   available
Mem:       16379476     9126884     3462688      148480     3789904     5123572
Swap:             0           0           0
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
$ free
              total        used        free      shared  buff/cache   available
Mem:       16379476     1694528    14106552      149772      578396    14256428
Swap:             0           0           0
$ sudo qemu.sh -m 10240  && echo success || echo failed
success