fork: share template mem-file via symlink for fan-out forks#214
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sjmiller609 wants to merge 1 commit intohypeship/templatesfrom
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fork: share template mem-file via symlink for fan-out forks#214sjmiller609 wants to merge 1 commit intohypeship/templatesfrom
sjmiller609 wants to merge 1 commit intohypeship/templatesfrom
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When ForkInstanceRequest.TemplateID is set, the fork resolves the source
instance from the templates registry, skips the per-fork mem-file copy,
and installs a symlink to the template's snapshot mem-file instead.
firecracker mmaps the symlinked mem-file MAP_PRIVATE during restore, so
many concurrent forks COW from the same backing file rather than each
holding a private copy.
Also wires:
- templateGuard on Start/Restore so a template parent is never resumed
while live forks share its mem-file.
- Refcount lifecycle: bump on fork creation, decrement on fork delete.
- Delete safety: deleting a template instance refuses while ForkCount>0
via templates.ErrInUse.
- forkvm.CopyOptions.SkipRelPaths so callers can opt out of specific
files in the source dir without breaking the existing copy semantics.
This was referenced May 8, 2026
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Stacked on: #213 (templates) — review/merge that first.
Summary
forkInstanceFromStoppedOrStandbysymlinks<dst>/snapshots/snapshot-latest/memoryto the template's canonical mem-file path.forkvm.CopyOptions{SkipRelPaths}so the directory copy walks past the mem-file without touching it.ForkCountbefore releasing cleanup so a partial fork rolls back the refcount.ForkInstanceRequest.TemplateIDand a pre-lock template→source resolution step so the right instance lock is held.Why
The template's
memoryfile is read-only and ~equal to guest RAM; copying it per fork burns most of fan-out's wall-clock budget. Sharing via symlink is the cheapest legal MAP_PRIVATE-equivalent today and the building block PR 5's UFFD server replaces with a real shared mapping later.Test plan
go test ./lib/forkvm/...(covers theSkipRelPathsoption)go test ./lib/instances/... -run "TestForkInstance|TestValidateFork|TestForkRequest"<fork>/snapshots/snapshot-latest/memoryis a symlink to the template's mem-fileErrInUse)🤖 Generated with Claude Code