sys::mman: adding MREMAP_DONTUNMAP flag for mremap on Linux.

Is meant to be used with `MREMAP_MAYMOVE` and allow to not unmapping
`old_address`.

changelog/2555.added.md

@@ -0,0 +1 @@
+Add `MremapFlags::MREMAP_DONTUNMAP` to `sys::mman::mremap` for linux target.

src/sys/mman.rs

@@ -205,6 +205,10 @@ libc_bitflags! {
         /// Place the mapping at exactly the address specified in `new_address`.
         #[cfg(target_os = "linux")]
         MREMAP_FIXED;
+        /// Works in conjunction with `MREMAP_MAYMOVE` but does not unmap `old_address`.
+        /// Note that, in this case, `old_size` and `new_size` must be the same.
+        #[cfg(target_os = "linux")]
+        MREMAP_DONTUNMAP;
         /// Place the mapping at exactly the address specified in `new_address`.
         #[cfg(target_os = "netbsd")]
         MAP_FIXED;

test/sys/test_mman.rs

@@ -119,3 +119,40 @@ fn test_mremap_shrink() {
     // The first KB should still be accessible and have the old data in it.
     assert_eq!(slice[ONE_K - 1], 0xFF);
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_mremap_dontunmap() {
+    use nix::libc::size_t;
+    use nix::sys::mman::{mremap, MRemapFlags};
+    use std::num::NonZeroUsize;
+    use std::ptr::NonNull;
+
+    const ONE_K: size_t = 1024;
+    let one_k_non_zero = NonZeroUsize::new(ONE_K).unwrap();
+
+    let slice: &mut [u8] = unsafe {
+        let mem = mmap_anonymous(
+            None,
+            one_k_non_zero,
+            ProtFlags::PROT_READ | ProtFlags::PROT_WRITE,
+            MapFlags::MAP_PRIVATE,
+        )
+        .unwrap();
+        std::slice::from_raw_parts_mut(mem.as_ptr().cast(), ONE_K)
+    };
+
+    // because we do not unmap `slice`, `old_size` and `new_size`
+    // need to be equal or `EINVAL` is set.
+    let _new_slice: &mut [u8] = unsafe {
+        let mem = mremap(
+            NonNull::from(&mut slice[..]).cast(),
+            ONE_K,
+            ONE_K,
+            MRemapFlags::MREMAP_MAYMOVE | MRemapFlags::MREMAP_DONTUNMAP,
+            None,
+        )
+        .unwrap();
+        std::slice::from_raw_parts_mut(mem.cast().as_ptr(), 10 * ONE_K)
+    };
+}