[PATCH v6 2/2] rust: zpool: add abstraction for zpool drivers

Vitaly Wool
2025-09-23T10:27:12Z

zpool is a common thin API for memory storage pool implementations.
A backend implementing this API will be used to store compressed
memory objects, e. g. for zswap, the lightweight compressed cache for
swap pages.

This patch provides the interface to use zpool in Rust kernel code,
thus enabling Rust implementations of zpool allocators for zswap. Now
that zpool API doesn't provide runtime registration option and presumes
compile time selection of an allocation backend, an allocator is
expected to call DECLARE_ZPOOL! macro to register itself during compile
time. Please also see the example in zpool.rs for more details.

Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.se>
---
rust/bindings/bindings_helper.h | 1 +
rust/kernel/lib.rs | 2 +
rust/kernel/zpool.rs | 366 ++++++++++++++++++++++++++++++++
3 files changed, 369 insertions(+)
create mode 100644 rust/kernel/zpool.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 84d60635e8a9..f0c4c454882b 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -75,6 +75,7 @@
#include <linux/wait.h>
#include <linux/workqueue.h>
#include <linux/xarray.h>
+#include <linux/zpool.h>
#include <trace/events/rust_sample.h>

#if defined(CONFIG_DRM_PANIC_SCREEN_QR_CODE)
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 6b0a5689669f..4d5b0cc881d8 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -130,6 +130,8 @@
pub mod uaccess;
pub mod workqueue;
pub mod xarray;
+#[cfg(CONFIG_ZPOOL_ALLOCATOR)]
+pub mod zpool;

#[doc(hidden)]
pub use bindings;
diff --git a/rust/kernel/zpool.rs b/rust/kernel/zpool.rs
new file mode 100644
index 000000000000..53a0dc0607e6
--- /dev/null
+++ b/rust/kernel/zpool.rs
@@ -0,0 +1,366 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Implementation of Rust interface towards zpool API.
+
+use crate::{error::Result, kernel::alloc::Flags, str::CString, types::ForeignOwnable};
+use core::ptr::NonNull;
+use kernel::alloc::NumaNode;
+
+/// The Rust representation of zpool handle.
+pub struct ZpoolHandle(usize);
+
+impl ZpoolHandle {
+ /// Create `ZpoolHandle` from the raw representation.
+ pub fn from_raw(h: usize) -> Self {
+ Self(h)
+ }
+
+ /// Get the raw representation of the handle.
+ pub fn as_raw(self) -> usize {
+ self.0
+ }
+}
+
+/// Zpool API.
+///
+/// The [`ZpoolDriver`] trait serves as an interface for Zpool drivers implemented in Rust.
+/// Such drivers implement memory storage pools in accordance with the zpool API.
+///
+/// # Example
+///
+/// A zpool driver implementation which uses KVec of 2**n sizes, n = 6, 7, ..., PAGE_SHIFT.
+/// Every zpool object is packed into a KVec that is sufficiently large, and n (the
+/// denominator) is saved in the least significant bits of the handle, which is guaranteed
+/// to be at least 2**6 aligned by kmalloc.
+///
+/// ```
+/// use core::ptr::{NonNull, copy_nonoverlapping};
+/// use core::sync::atomic::{AtomicU64, Ordering};
+/// use kernel::alloc::{Flags, flags, KBox, KVec, NumaNode};
+/// use kernel::page::PAGE_SHIFT;
+/// use kernel::prelude::EINVAL;
+/// use kernel::str::CString;
+/// use kernel::zpool::*;
+///
+/// struct MyZpool {
+/// name: CString,
+/// bytes_used: AtomicU64,
+/// }
+///
+/// struct MyZpoolDriver;
+///
+/// impl ZpoolDriver for MyZpoolDriver {
+/// type Pool = KBox<MyZpool>;
+///
+/// fn create(name: CString, gfp: Flags) -> Result<KBox<MyZpool>> {
+/// let my_pool = MyZpool { name, bytes_used: AtomicU64::new(0) };
+/// let pool = KBox::new(my_pool, gfp)?;
+///
+/// pr_debug!("Pool {:?} created\n", pool.name);
+/// Ok(pool)
+/// }
+///
+/// fn malloc(pool: &MyZpool, size: usize, _gfp: Flags, _nid: NumaNode) -> Result<ZpoolHandle> {
+/// let pow = size.next_power_of_two().trailing_zeros().max(6);
+/// match pow {
+/// 0 => Err(EINVAL),
+/// m if m > PAGE_SHIFT as u32 => Err(ENOSPC),
+/// _ => {
+/// let vec = KVec::<u8>::with_capacity(1 << pow, GFP_KERNEL)?;
+/// let (ptr, _len, _cap) = vec.into_raw_parts();
+///
+/// // We assume that kmalloc-64, kmalloc-128 etc. kmem caches will be used for
+/// // our allocations, so it's actually `1 << pow` bytes that we have consumed.
+/// pool.bytes_used.fetch_add(1 << pow, Ordering::Relaxed);
+///
+/// // `kmalloc` guarantees that an allocation of size x*2^n is 2^n aligned.
+/// // Therefore the 6 lower bits are zeros and we can use these to store `pow`.
+/// Ok(ZpoolHandle::from_raw(ptr as usize | (pow as usize - 6)))
+/// }
+/// }
+/// }
+///
+/// unsafe fn free(pool: &MyZpool, handle: ZpoolHandle) {
+/// let h = handle.as_raw();
+/// let n = (h & 0x3F) + 6;
+/// let uptr = h & !0x3F;
+///
+/// // SAFETY:
+/// // - we derive `uptr` from handle by zeroing 6 lower bits where we store the power
+/// // denominator for the vector capacity. As noted above, the result will be exactly the
+/// // pointer to the area allocated by `KVec`. Thus, uptr is a valid pointer pointing to
+/// // the vector allocated by `alloc` function above.
+/// // - 1 << n == capacity and is coming from the first 6 bits of handle.
+/// let vec = unsafe { KVec::<u8>::from_raw_parts(uptr as *mut u8, 0, 1 << n) };
+/// drop(vec);
+/// pool.bytes_used.fetch_sub(1 << n, Ordering::Relaxed);
+/// }
+///
+/// unsafe fn read_begin(_pool: &MyZpool, handle: ZpoolHandle) -> NonNull<u8> {
+/// let uptr = handle.as_raw() & !0x3F;
+/// // SAFETY:
+/// // - we derive `uptr` from handle by zeroing 6 lower bits where we store the power
+/// // denominator for the vector capacity. As noted above, the result will be exactly the
+/// // pointer to the area allocated by `KVec`. Thus, uptr is a valid pointer pointing to
+/// // the vector allocated by `alloc` function above.
+/// unsafe { NonNull::new_unchecked(uptr as *mut u8) }
+/// }
+///
+/// unsafe fn read_end(_pool: &MyZpool, _handle: ZpoolHandle, _handle_mem: NonNull<u8>) {}
+///
+/// unsafe fn write(_p: &MyZpool, handle: ZpoolHandle, h_mem: NonNull<u8>, mem_len: usize) {
+/// let uptr = handle.as_raw() & !0x3F;
+/// // SAFETY:
+/// // - `h_mem` is a valid non-null pointer provided by zswap.
+/// // - `uptr` is derived from handle by zeroing 6 lower bits where we store the power
+/// // denominator for the vector capacity. As noted above, the result will be exactly the
+/// // pointer to the area allocated by `KVec`. Thus, uptr is a valid pointer pointing to
+/// // the vector allocated by `alloc` function above.
+/// unsafe {
+/// copy_nonoverlapping(h_mem.as_ptr().cast(), uptr as *mut c_void, mem_len)
+/// };
+/// }
+///
+/// fn total_pages(pool: &MyZpool) -> u64 {
+/// pool.bytes_used.load(Ordering::Relaxed) >> PAGE_SHIFT
+/// }
+/// }
+///
+/// // Uncomment this for compile time registration (disabled to avoid build error when KUNIT
+/// // tests and zsmalloc are enabled):
+/// // kernel::DECLARE_ZPOOL!(MyZpoolDriver);
+/// ```
+///
+pub trait ZpoolDriver {
+ /// Opaque Rust representation of `struct zpool`.
+ type Pool: ForeignOwnable;
+
+ /// Create a pool.
+ fn create(name: CString, gfp: Flags) -> Result<Self::Pool>;
+
+ /// Allocate an object of `size` bytes from `pool`, with the allocation flags `gfp` and
+ /// preferred NUMA node `nid`. If the allocation is successful, an opaque handle is returned.
+ fn malloc(
+ pool: <Self::Pool as ForeignOwnable>::Borrowed<'_>,
+ size: usize,
+ gfp: Flags,
+ nid: NumaNode,
+ ) -> Result<ZpoolHandle>;
+
+ /// Free an object previously allocated from the `pool`, represented by `handle`.
+ ///
+ /// # Safety
+ ///
+ /// - `handle` must be a valid handle previously returned by `malloc`.
+ /// - `handle` must not be used any more after the call to `free`.
+ unsafe fn free(pool: <Self::Pool as ForeignOwnable>::Borrowed<'_>, handle: ZpoolHandle);
+
+ /// Make all the necessary preparations for the caller to be able to read from the object
+ /// represented by `handle` and return a valid pointer to that object's memory to be read.
+ ///
+ /// # Safety
+ ///
+ /// - `handle` must be a valid handle previously returned by `malloc`.
+ /// - `read_end` with the same `handle` must be called for each `read_begin`.
+ unsafe fn read_begin(
+ pool: <Self::Pool as ForeignOwnable>::Borrowed<'_>,
+ handle: ZpoolHandle,
+ ) -> NonNull<u8>;
+
+ /// Finish reading from a previously allocated `handle`. `handle_mem` must be the pointer
+ /// previously returned by `read_begin`.
+ ///
+ /// # Safety
+ ///
+ /// - `handle` must be a valid handle previously returned by `malloc`.
+ /// - `handle_mem` must be the pointer previously returned by `read_begin`.
+ unsafe fn read_end(
+ pool: <Self::Pool as ForeignOwnable>::Borrowed<'_>,
+ handle: ZpoolHandle,
+ handle_mem: NonNull<u8>,
+ );
+
+ /// Write to the object represented by a previously allocated `handle`. `handle_mem` points
+ /// to the memory to copy data from, and `mem_len` defines the length of the data block to
+ /// be copied.
+ ///
+ /// # Safety
+ ///
+ /// - `handle` must be a valid handle previously returned by `malloc`.
+ /// - `handle_mem` must be a valid pointer into the allocated memory aread represented by
+ /// `handle`.
+ /// - `handle_mem + mem_len - 1` must not point outside the allocated memory area.
+ unsafe fn write(
+ pool: <Self::Pool as ForeignOwnable>::Borrowed<'_>,
+ handle: ZpoolHandle,
+ handle_mem: NonNull<u8>,
+ mem_len: usize,
+ );
+
+ /// Get the number of pages used by the `pool`.
+ fn total_pages(pool: <Self::Pool as ForeignOwnable>::Borrowed<'_>) -> u64;
+}
+
+/// Provide a zpool allocator to zswap
+#[macro_export]
+macro_rules! DECLARE_ZPOOL {
+ ($t: ident) => {
+ use core::ptr::null_mut;
+ use kernel::error::from_result;
+ use kernel::types::ForeignOwnable;
+
+ const __LOG_PREFIX: &[u8] = b"zpool_rust\0";
+
+ fn handle_from_result<T, F>(f: F) -> T
+ where
+ T: From<usize>,
+ F: FnOnce() -> Result<T>,
+ {
+ match f() {
+ Ok(v) => v,
+ Err(e) => T::from(0),
+ }
+ }
+
+ /// Create a pool.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_create_pool(name: *const c_uchar) -> *mut c_void {
+ match (|| -> Result<<$t as ZpoolDriver>::Pool> {
+ // SAFETY: the memory pointed to by name is guaranteed by `zpool` to be a valid
+ // string.
+ let name_r = unsafe { CStr::from_char_ptr(name).to_cstring() }?;
+ $t::create(name_r, flags::GFP_KERNEL)
+ })() {
+ Err(_) => null_mut(),
+ Ok(pool) => <$t as ZpoolDriver>::Pool::into_foreign(pool),
+ }
+ }
+
+ /// Destroy the pool.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_destroy_pool(pool: *mut c_void) {
+ // SAFETY: The pointer originates from an `into_foreign` call.
+ drop(unsafe { <$t as ZpoolDriver>::Pool::from_foreign(pool) })
+ }
+
+ /// Allocate an object of `size` bytes from `pool`, with the allocation flags `gfp` and
+ /// preferred NUMA node `nid`. If the allocation is successful, an opaque handle
+ /// is returned.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_malloc(
+ pool: *mut c_void,
+ size: usize,
+ gfp: u32,
+ nid: c_int,
+ ) -> c_ulong {
+ // SAFETY: The pointer originates from an `into_foreign` call. If `pool` is passed to
+ // `from_foreign`, then that happens in `zpool_destroy_pool` which will not be called
+ // during this method.
+ let pool = unsafe { <$t as ZpoolDriver>::Pool::borrow(pool) };
+ handle_from_result(|| {
+ let the_nid = match nid {
+ kernel::bindings::NUMA_NO_NODE => NumaNode::NO_NODE,
+ _ => NumaNode::new(nid)?,
+ };
+ let h = $t::malloc(
+ pool,
+ size,
+ flags::GFP_NOWAIT | flags::__GFP_HIGHMEM,
+ the_nid,
+ )?;
+ Ok(h.as_raw())
+ })
+ }
+
+ /// Free an object previously allocated from the `pool`, represented by `handle`.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_free(pool: *mut c_void, handle: usize) {
+ // SAFETY: The pointer originates from an `into_foreign` call. If `pool` is passed to
+ // `from_foreign`, then that happens in `zpool_destroy_pool` which will not be called
+ // during this method.
+ let pool = unsafe { <$t as ZpoolDriver>::Pool::borrow(pool) };
+
+ // SAFETY:
+ // - the caller (`zswap`) guarantees that `handle` is a valid handle previously
+ // allocated by `malloc`.
+ // - the caller (`zswap`) guarantees that it will not call any other function with this
+ // `handle` as a parameter after this call.
+ unsafe { $t::free(pool, ZpoolHandle::from_raw(handle)) }
+ }
+
+ /// Make all the necessary preparations for the caller to be able to read from the object
+ /// represented by `handle` and return a valid pointer to that object's memory to be read.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_obj_read_begin(
+ pool: *mut c_void,
+ handle: usize,
+ _local_copy: *mut c_void,
+ ) -> *mut c_void {
+ // SAFETY: The pointer originates from an `into_foreign` call. If `pool` is passed to
+ // `from_foreign`, then that happens in `zpool_destroy_pool` which will not be called
+ // during this method.
+ let pool = unsafe { <$t as ZpoolDriver>::Pool::borrow(pool) };
+
+ // SAFETY: the caller (`zswap`) guarantees that `handle` is a valid handle previously
+ // allocated by `malloc`.
+ let non_null_ptr = unsafe { $t::read_begin(pool, ZpoolHandle::from_raw(handle)) };
+ non_null_ptr.as_ptr().cast()
+ }
+
+ /// Finish reading from a previously allocated `handle`. `handle_mem` must be the pointer
+ /// previously returned by `read_begin`.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_obj_read_end(
+ pool: *mut c_void,
+ handle: usize,
+ handle_mem: *mut c_void,
+ ) {
+ // SAFETY: The pointer originates from an `into_foreign` call. If `pool` is passed to
+ // `from_foreign`, then that happens in `zpool_destroy_pool` which will not be called
+ // during this method.
+ let pool = unsafe { <$t as ZpoolDriver>::Pool::borrow(pool) };
+
+ // SAFETY: `handle_mem` is guaranteed to be non-null by the caller (`zswap`).
+ let handle_mem_ptr = unsafe { NonNull::new_unchecked(handle_mem.cast()) };
+
+ // SAFETY: the caller (`zswap`) guarantees that `handle` is a valid handle previously
+ // allocated by `malloc`.
+ unsafe { $t::read_end(pool, ZpoolHandle::from_raw(handle), handle_mem_ptr) }
+ }
+
+ /// Write to the object represented by a previously allocated `handle`. `handle_mem` points
+ /// to the memory to copy data from, and `mem_len` defines the length of the data block to
+ /// be copied.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_obj_write(
+ pool: *mut c_void,
+ handle: usize,
+ handle_mem: *mut c_void,
+ mem_len: usize,
+ ) {
+ // SAFETY: The pointer originates from an `into_foreign` call. If `pool` is passed to
+ // `from_foreign`, then that happens in `zpool_destroy_pool` which will not be called
+ // during this method.
+ let pool = unsafe { <$t as ZpoolDriver>::Pool::borrow(pool) };
+
+ // SAFETY: `handle_mem` is guaranteed to be non-null by the caller (`zswap`).
+ let handle_mem_ptr = unsafe { NonNull::new_unchecked(handle_mem.cast()) };
+
+ // SAFETY: the caller (`zswap`) guarantees that `handle` is a valid handle previously
+ // allocated by `malloc`.
+ unsafe {
+ $t::write(pool, ZpoolHandle::from_raw(handle), handle_mem_ptr, mem_len);
+ }
+ }
+
+ /// Get the number of pages used by the `pool`.
+ #[no_mangle]
+ pub unsafe extern "C" fn zpool_get_total_pages(pool: *mut c_void) -> u64 {
+ // SAFETY: The pointer originates from an `into_foreign` call. If `pool` is passed to
+ // `from_foreign`, then that happens in `zpool_destroy_pool` which will not be called
+ // during this method.
+ let pool = unsafe { <$t as ZpoolDriver>::Pool::borrow(pool) };
+ $t::total_pages(pool)
+ }
+ };
+}
--
2.39.2