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OverlayFS broken on NixOS Kernel 4.19 and 4.20 #54509

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OverlayFS broken on NixOS Kernel 4.19 and 4.20#54509
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1.severity: blockerThis is preventing another PR or issue from being completed6.topic: nixosIssues or PRs affecting NixOS modules, or package usability issues specific to NixOS
Milestone
19.03
@bachp

Description

@bachp
bachp
opened on Jan 23, 2019

Issue description

When using overlayfs with NixOS and Kernel 4.19 it is not possible to overwrite a file that already exists in the lower directory with new content. Instead if attempted the file appears empty. (See how to reproduce for details).

It seems like the revert of an upstream commit in 4.19 #52942 breaks something fundamentally in overlayfs.

EDIT: Reverting #52942 fixes the behavior. But I'm unable to run the tests as the VM kernel pancis :(

This also affects docker if used with one of the overlay drivers.

Steps to reproduce

Via the test in #54508

or

Manually:

mkdir -p /tmp/mnt/upper /tmp/mnt/lower /tmp/mnt/work /tmp/mnt/merged

# Create an existing file in the lower directory
echo 'Existing' > /tmp/mnt/lower/existing.txt

# Mount overlayfs
mount -t overlay overlay -o lowerdir=/tmp/mnt/lower,upperdir=/tmp/mnt/upper,workdir=/tmp/mnt/work /tmp/mnt/merged
 
# Prints "Existing" - OK
cat /tmp/mnt/merged/existing.txt

# Write some new content
"echo 'New' > /tmp/mnt/merged/existing.txt",

# Prints "" (empty) but it should be New - FAIL
cat /tmp/mnt/merged/existing.txt

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

 - system: `"x86_64-linux"`
 - host os: `Linux 4.19.16, NixOS, 19.03.git.5d42db2 (Koi)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2`
 - channels(root): `"nixos-17.03pre95306.a24728f"`
 - channels(pascal): `"nixos-19.03pre167251.20343f0ab47"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

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    1.severity: blockerThis is preventing another PR or issue from being completed6.topic: nixosIssues or PRs affecting NixOS modules, or package usability issues specific to NixOS

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