mpk: add an example testing the memory limits (#7609)

* mpk: allow checking for MPK without a config instance

It is inconvenient to have to construct a `PoolingAllocationConfig` in
order to check if memory protection keys are available. This removes
the unused `&self` restriction.

* mpk: improve logging of calculated slab layout

When double-checking the slab layout calculations it is quite convenient
to see the total slab size. This helps in correlating with mapped
regions.

* mpk: add an example testing the memory limits

This adds an example that can be run with `cargo run --example mpk`. Not
only does the example demonstrate how to build a pool-allocated engine
that uses MPK, it performs an exponential search to find the maximum
number of slots the system can support, with and without MPK.

* review: document Linux requirement

* review: `env_logger::init`

* review: replace `proc-maps` with manual parsing

* vet: audit `bytesize`

* fix: provide `main` for non-Linux systems

* fix: move `cfg` to avoid unused code

Cargo.toml

@@ -89,6 +89,7 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 walkdir = { workspace = true }
 test-programs-artifacts = { workspace = true }
+bytesize = "1.3.0"
 
 [target.'cfg(windows)'.dev-dependencies]
 windows-sys = { workspace = true, features = ["Win32_System_Memory"] }

crates/runtime/src/instance/allocator/pooling/memory_pool.rs

@@ -189,7 +189,10 @@ impl MemoryPool {
         // region to start--`PROT_NONE`.
         let constraints = SlabConstraints::new(&config.limits, tunables, pkeys.len())?;
         let layout = calculate(&constraints)?;
-        log::debug!("creating memory pool: {constraints:?} -> {layout:?}");
+        log::debug!(
+            "creating memory pool: {constraints:?} -> {layout:?} (total: {})",
+            layout.total_slab_bytes()?
+        );
         let mut mapping = Mmap::accessible_reserved(0, layout.total_slab_bytes()?)
             .context("failed to create memory pool mapping")?;
 

crates/wasmtime/src/config.rs

@@ -2409,7 +2409,7 @@ impl PoolingAllocationConfig {
     /// same method that [`MpkEnabled::Auto`] does. See
     /// [`PoolingAllocationConfig::memory_protection_keys`] for more
     /// information.
-    pub fn are_memory_protection_keys_available(&self) -> bool {
+    pub fn are_memory_protection_keys_available() -> bool {
         mpk::is_supported()
     }
 }