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Incorrect memory accounting when using tmpfs volume

I have a workload where I want to use a container to write data to a tmpfs volume mounted from the host with -v. I would expect that data written to the volume would not count towards Docker’s memory limit for the container doing the write. However, data written to the host-mounted volume seems to count towards the container’s memory limit. Is this correct behavior? Is there any way to have a container write more data than its memory limit to the memory of the host machine?

Reproduction steps from a freshly-provisioned EC2 machine running amazon linux:

sudo yum install -y docker
# Mount a 5GB tmpfs 
sudo mkdir /mnt/tmpfs && sudo mount -t tmpfs -o size=5G tmpfs /mnt/tmpfs
# Get some data. This file is 3GB
wget --no-check-certificate
sudo service docker start
sudo docker run --rm --memory=2G -it -v /mnt/tmpfs:/mnt/tmpfs -v ~/data:/data ubuntu /bin/bash
# Inside the container now
cp /data /mnt/tmpfs
# The cp command gets killed

If we don’t set the 2G memory limit, the copy will finish successfully.