Ian Klemm 39e253ff9c block: qcow: Preserve WRITE_ZEROES with backing files ...

QCOW2 empty L2 entries in an overlay mean that reads fall
through to the backing file. Reusing the punch_hole path for
WRITE_ZEROES therefore turns a full-cluster zero operation on an
unallocated overlay cluster into backing data exposure.

Keep discard/punch_hole behavior unchanged, but let WRITE_ZEROES
request a logical-zero marker when the image has a backing file.
ZERO_FLAG entries now read as zeros in both the legacy QcowFile
path and the shared runtime metadata path. Partial writes after
such entries seed new clusters from zeros instead of backing data.

Treat ZERO_FLAG entries as logical holes for SEEK_HOLE/SEEK_DATA.
Empty overlay entries with a backing file still report data because
the data exists in the backing file.

Avoid cluster-sized userspace zero buffers when materializing
zero-flagged clusters by zeroing the allocated host range directly.
This keeps recycled clusters safe without making partial writes
allocate large zero-filled Vecs.

Add regression coverage for legacy QcowFile, QcowSync, direct I/O,
QcowAsync/io_uring overlay paths, and a large-cluster partial-write
case.

Assisted-by: Codex:GPT-5

Signed-off-by: Ian Klemm <hi@ianklemm.de>

2026-05-22 14:26:04 +00:00

..

src

block: qcow: Preserve WRITE_ZEROES with backing files

2026-05-22 14:26:04 +00:00

Cargo.toml

block: fix cargo test -p block

2026-05-09 14:10:25 +00:00