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vmm: open backing file read-only when not shared

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When restoring from snapshot with shared=false, write access to the
backing file is not required. Opening it read-only allows restore to
succeed on read-only media and overlay lower layers while preserving
MAP_PRIVATE semantics.

Signed-off-by: Rowen-Ye <rowenye1@gmail.com>
This commit is contained in:
Rowen-Ye 2026-02-06 19:07:43 +08:00 • committed by Rob Bradford
parent 258f826027
commit 2c2f5d2431
1 changed files with 7 additions and 3 deletions
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10
vmm/src/memory_manager.rs
View file

@ -1356,13 +1356,17 @@ impl MemoryManager {
Ok(FileOffset::new(f, 0))
}
fn open_backing_file(backing_file: &PathBuf, file_offset: u64) -> Result<FileOffset, Error> {
fn open_backing_file(
backing_file: &PathBuf,
file_offset: u64,
shared: bool,
) -> Result<FileOffset, Error> {
if backing_file.is_dir() {
Err(Error::DirectoryAsBackingFileForMemory)
} else {
let f = OpenOptions::new()
.read(true)
.write(true)
.write(shared)
.open(backing_file)
.map_err(Error::SharedFileCreate)?;
@ -1397,7 +1401,7 @@ impl MemoryManager {
} else {
mmap_flags |= libc::MAP_PRIVATE;
}
Some(Self::open_backing_file(backing_file, file_offset)?)
Some(Self::open_backing_file(backing_file, file_offset, shared)?)
} else if shared || hugepages {
// For hugepages we must also MAP_SHARED otherwise we will trigger #4805
// because the MAP_PRIVATE will trigger CoW against the backing file with