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Activation touches file bind mount targets even when they already exist in the new lower layer. This copies them into the persistent upperdir with origin handles that refer to that generation's metadata image. Only create missing targets. Extend the mutable overlay VM test to check that a bind mount over a declared regular file survives two generation switches without copying its target into the upperdir. Assisted-by: Codex (OpenAI GPT-6.1-Sol)
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October 20, 2026 10:13
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During a mutable
/etcoverlay switch, activation callstouchon file bind mount targets even when they already exist in the new lower layer (code).The
touchresults in a timestamp update, which triggers overlayfs copy-up. The copied-up file persists in the upperdir with an origin handle that refers to that generation's lower inode.We should create the target only when it is missing to avoid this unnecessary copy-up (updated code).
The regression test declares an empty regular file in the base configuration, so the lowerdir provides it in all three generations. It bind-mounts a regular file containing
preservedover/etc/declared-filemount.On switch,
-f "$mountPoint"sees the regular bind-mounted source and returns true. Before transferring that mount, the old code touches$tmpMountPoint, which still exposes the empty file from the new lowerdir. This creates the unwanted upperdir copy.After each of two generation switches, the test checks:
/etc/declared-filemountreturnspreserved, confirming that the bind mount was transferred./.rw-etc/upper/declared-filemountdoes not exist, confirming that activation did not copy up the lowerdir placeholder.The first switch is enough to reproduce the bug; the second checks the same properties along the existing test's next switch. The original file-mount case binds
/dev/null, a character device. Its-fcheck returns false, so it skips thetouchbranch and does not cover this bug.Related to #533343 (mount-point handling) and #437424. This fix is a small change and can be applied independently of the overlay switch rewrite.
Validation on x86_64-linux:
nixfmt --checkon both changed files andgit diff --checkpass.nix-build nixos/release.nix -A tests.activation-etc-overlay-mutable.x86_64-linux --no-out-link --max-jobs 2 --cores 2AI disclosure: Codex with OpenAI GPT-6.1-Sol assisted with the code, regression test, commit message, and PR description. Validation used the NixOS VM test framework and nixfmt.
Things done
./result/bin/.nixpkgs-reviewon this PR. See nixpkgs-review usage.passthru.tests.