turbo-persistence: remove Unmergeable mmap advice (#91713)

Remove the `MADV_UNMERGEABLE` (Unmergeable) mmap advice from
`turbo-persistence`'s `advise_mmap_for_persistence` helper in
`mmap_helper.rs`.

The WSL (Windows Subsystem for Linux) kernel does not support
`MADV_UNMERGEABLE` and returns an error when it is applied, causing
turbo-persistence to fail on WSL. See:
https://x.com/von_cronen/status/2034944304712392891

Additionally, `MADV_UNMERGEABLE` is already the **default** state for
memory pages — KSM (Kernel Same-page Merging) is opt-in via
`MADV_MERGEABLE`. Explicitly setting `MADV_UNMERGEABLE` on pages that
were never marked mergeable is a no-op on standard kernels, so the call
provides no benefit while breaking WSL users.

The `MADV_DONTFORK` advice is retained, as it has a clear correctness
benefit (prevents mmap regions from being inherited by child processes
on `fork()`).

Removed the `mmap.advise(memmap2::Advice::Unmergeable)` call and its
associated doc comment from
`turbopack/crates/turbo-persistence/src/mmap_helper.rs`.

Co-authored-by: Tobias Koppers <sokra@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 2 people

turbopack/crates/turbo-persistence/src/mmap_helper.rs

@@ -2,12 +2,11 @@
 ///
 /// - `DontFork`: prevents mmap regions from being copied into child processes on `fork()`, avoiding
 ///   unnecessary memory duplication and potential SIGBUS.
-/// - `Unmergeable`: opts pages out of KSM (Kernel Same-page Merging) since our data is unique
-///   compressed content that won't benefit from deduplication scanning.
 #[cfg(target_os = "linux")]
 pub fn advise_mmap_for_persistence(mmap: &memmap2::Mmap) -> anyhow::Result<()> {
-    mmap.advise(memmap2::Advice::DontFork)?;
-    mmap.advise(memmap2::Advice::Unmergeable)?;
+    use anyhow::Context;
+    mmap.advise(memmap2::Advice::DontFork)
+        .context("Failed to advise mmap DontFork")?;
     Ok(())
 }