rust: alloc: add __GFP_HIGHMEM flag

Make it possible to allocate memory that doesn't need to mapped into the
kernel's address space. This flag is useful together with
Page::alloc_page [1].

Rust Binder needs this for the memory that holds incoming transactions
for each process. Each process will have a few megabytes of memory
allocated with this flag, which is mapped into the process using
vm_insert_page. When the kernel copies data for an incoming transaction
into a process's memory region, it will use kmap_local_page to
temporarily map pages that are being modified. There is no need for them
to take up address space in the kernel when the kernel is not writing an
incoming transaction into the page.

Link: https://lore.kernel.org/all/20240528-alice-mm-v7-4-78222c31b8f4@google.com/ [1]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240607-highmem-v1-1-d18c5ca4072f@google.comSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindings/bindings_helper.h

@@ -25,3 +25,4 @@ const gfp_t RUST_CONST_HELPER_GFP_KERNEL = GFP_KERNEL;
 const gfp_t RUST_CONST_HELPER_GFP_KERNEL_ACCOUNT = GFP_KERNEL_ACCOUNT;
 const gfp_t RUST_CONST_HELPER_GFP_NOWAIT = GFP_NOWAIT;
 const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
+const gfp_t RUST_CONST_HELPER___GFP_HIGHMEM = ___GFP_HIGHMEM;

rust/kernel/alloc.rs

@@ -52,6 +52,14 @@ pub mod flags {
     /// This is normally or'd with other flags.
     pub const __GFP_ZERO: Flags = Flags(bindings::__GFP_ZERO);
 
+    /// Allow the allocation to be in high memory.
+    ///
+    /// Allocations in high memory may not be mapped into the kernel's address space, so this can't
+    /// be used with `kmalloc` and other similar methods.
+    ///
+    /// This is normally or'd with other flags.
+    pub const __GFP_HIGHMEM: Flags = Flags(bindings::__GFP_HIGHMEM);
+
     /// Users can not sleep and need the allocation to succeed.
     ///
     /// A lower watermark is applied to allow access to "atomic reserves". The current