disk_blockdevice: support discard for file-backed disks - #4088
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File-backed disks previously ignored guest UNMAP requests entirely: the only discard path was the BLKDISCARD ioctl, which is valid only on block devices, so regular files reported no discard support and silently dropped every unmap. Service discard on files by punching a hole with fallocate (PUNCH_HOLE | KEEP_SIZE), submitted asynchronously through io-uring so it sits on the same completion path as reads, writes, and flush rather than blocking a worker thread. Capability is detected at open time with a non-destructive probe that punches a zero-length-effect hole past the end of the file; filesystems that cannot punch holes report no discard support and continue to treat unmap as a no-op. Because a punched hole deterministically reads back as zero, file-backed disks now report UnmapBehavior::Zeroes, letting the guest rely on unmapped ranges being zero. To preserve that guarantee, any failure of the file punch-hole path is propagated as an error rather than swallowed. The block device path and its behavior are unchanged.
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Pull request overview
Adds discard/UNMAP support for file-backed block devices by using hole-punching (fallocate(PUNCH_HOLE|KEEP_SIZE)) submitted via io-uring, and updates capability reporting so guests can rely on unmapped ranges reading back as zero when supported.
Changes:
- Detect and advertise discard support for regular files when hole punching is available and io-uring supports
IORING_OP_FALLOCATE. - Implement file-backed
unmapvia io-uring fallocate hole-punch, and reportUnmapBehavior::Zeroeswhen enabled. - Add an async test covering file-backed unmap semantics; wire in
test_with_tracingfor async tests.
Reviewed changes
Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| vm/devices/storage/disk_blockdevice/src/lib.rs | Add hole-punch probing, enable discard granularity for files, implement io-uring fallocate-based unmap, and add a test validating zero-readback behavior. |
| vm/devices/storage/disk_blockdevice/Cargo.toml | Add test_with_tracing to dev-dependencies for async tests. |
| Cargo.lock | Lockfile update for the new dev-dependency usage. |
sprt
approved these changes
Jul 30, 2026
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I can't test this e2e yet but code lgtm.
smalis-msft
approved these changes
Jul 30, 2026
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File-backed disks previously ignored guest UNMAP requests entirely: the only discard path was the BLKDISCARD ioctl, which is valid only on block devices, so regular files reported no discard support and silently dropped every unmap.
Service discard on files by punching a hole with fallocate (PUNCH_HOLE | KEEP_SIZE), submitted asynchronously through io-uring so it sits on the same completion path as reads, writes, and flush rather than blocking a worker thread. Capability is detected at open time with a non-destructive probe that punches a zero-length-effect hole past the end of the file; filesystems that cannot punch holes report no discard support and continue to treat unmap as a no-op.
Because a punched hole deterministically reads back as zero, file-backed disks now report UnmapBehavior::Zeroes, letting the guest rely on unmapped ranges being zero. To preserve that guarantee, any failure of the file punch-hole path is propagated as an error rather than swallowed. The block device path and its behavior are unchanged.