Auto merge of rust-lang#122309 - g-yziquel:issue-122262, r=saethlin

Use `MAP_PRIVATE` (not unsound-prone `MAP_SHARED`) Solves rust-lang#122262

Author: bors

compiler/rustc_data_structures/src/memmap.rs

@@ -18,8 +18,14 @@ impl Mmap {
  /// However in practice most callers do not ensure this, so uses of this function are likely unsound.
  #[inline]
  pub unsafe fn map(file: File) -> io::Result<Self> {
- // Safety: the caller must ensure that this is safe.
- unsafe { memmap2::Mmap::map(&file).map(Mmap) }
+ // By default, memmap2 creates shared mappings, implying that we could see updates to the
+ // file through the mapping. That would violate our precondition; so by requesting a
+ // map_copy_read_only we do not lose anything.
+ // This mapping mode also improves our support for filesystems such as cacheless virtiofs.
+ // For more details see https://github.com/rust-lang/rust/issues/122262
+ //
+ // SAFETY: The caller must ensure that this is safe.
+ unsafe { memmap2::MmapOptions::new().map_copy_read_only(&file).map(Mmap) }
  }
 }