Rob Bradford ee1d9dae34 vmm: memory_manager: Write snapshot file sparsely ...

For memfd-backed guest RAM regions, walk the backing fd extent by extent
via lseek(SEEK_DATA) / lseek(SEEK_HOLE) and write each populated extent
into the snapshot file's per-region slot via
std::os::unix::fs::FileExt::write_at. Pre-size the file with
set_len(total): on filesystems that support sparse files unwritten bytes
become real holes; on others the kernel zero-fills the allocation, which
is still byte-correct.

If set_len fails (some FUSE backends reject ftruncate-extend with
EOPNOTSUPP), fall back entirely to the dense write path which streams
bytes sequentially via write_volatile_to and never writes past the
growing EOF.

When the guest region has no backing file (anonymous mmap) or the
backing fd does not support SEEK_HOLE (hugetlbfs), fall back to the
dense write path on a per-region basis.

The on-disk byte stream is identical to the dense format from the
perspective of any reader using read/pread/mmap, so old readers see no
change.

Measured on a 4 GiB shared-memory VM (2 vCPUs, ~340 MiB touched), total
snapshot time as reported by the in-tree timing instrumentation:

  Before (dense):  ~2400ms, 4.0 GiB on disk
  After (sparse):  ~132ms,  340 MiB on disk (92% smaller, 18x faster)

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rob Bradford <rbradford@meta.com>

2026-05-01 15:40:50 +00:00

..

src

vmm: memory_manager: Write snapshot file sparsely

2026-05-01 15:40:50 +00:00

build.rs

build: Add "fuzzing" as a valid cfg(..) attribute

2024-05-08 08:10:28 +00:00

Cargo.toml

vmm: memory_manager: Add SEEK_DATA-based extent iterator

2026-05-01 15:40:50 +00:00