[RFC PATCH 21/24] erofs: introduce erofs_map_blocks alternative to C

[Yiyang Wu <toolmanp@tlmp.cc>]

This patch introduces erofs_map_blocks alternative written in Rust,
which will be hooked inside the erofs_iomap_begin.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/Makefile        |  2 +-
 fs/erofs/data.c          |  5 ++++
 fs/erofs/data_rs.rs      | 63 ++++++++++++++++++++++++++++++++++++++++
 fs/erofs/rust_bindings.h |  4 +++
 4 files changed, 73 insertions(+), 1 deletion(-)
 create mode 100644 fs/erofs/data_rs.rs

diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
index e086487971b6..219ddca0642e 100644
--- a/fs/erofs/Makefile
+++ b/fs/erofs/Makefile
@@ -9,4 +9,4 @@ erofs-$(CONFIG_EROFS_FS_ZIP_DEFLATE) += decompressor_deflate.o
 erofs-$(CONFIG_EROFS_FS_ZIP_ZSTD) += decompressor_zstd.o
 erofs-$(CONFIG_EROFS_FS_BACKED_BY_FILE) += fileio.o
 erofs-$(CONFIG_EROFS_FS_ONDEMAND) += fscache.o
-erofs-$(CONFIG_EROFS_FS_RUST) += super_rs.o inode_rs.o namei_rs.o dir_rs.o rust_helpers.o
+erofs-$(CONFIG_EROFS_FS_RUST) += super_rs.o inode_rs.o namei_rs.o dir_rs.o data_rs.o rust_helpers.o
diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 61debd799cf9..c9694661136b 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -293,7 +293,12 @@ static int erofs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	map.m_la = offset;
 	map.m_llen = length;
 
+#ifdef CONFIG_EROFS_FS_RUST
+	ret = erofs_map_blocks_rust(inode, &map);
+#else  
 	ret = erofs_map_blocks(inode, &map);
+#endif
+
 	if (ret < 0)
 		return ret;
 
diff --git a/fs/erofs/data_rs.rs b/fs/erofs/data_rs.rs
new file mode 100644
index 000000000000..ac34a9dd2079
--- /dev/null
+++ b/fs/erofs/data_rs.rs
@@ -0,0 +1,63 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+//! EROFS Rust Kernel Module Helpers Implementation
+//! This is only for experimental purpose. Feedback is always welcome.
+
+#[allow(dead_code)]
+#[allow(missing_docs)]
+pub(crate) mod rust;
+use core::ffi::*;
+use core::ptr::NonNull;
+
+use kernel::bindings::inode;
+use kernel::container_of;
+
+use rust::{erofs_sys::*, kinode::*, ksuperblock::*};
+
+#[repr(C)]
+struct ErofsBuf {
+    mapping: NonNull<c_void>,
+    page: NonNull<c_void>,
+    base: NonNull<c_void>,
+    kmap_type: c_int,
+}
+
+/// A helper sturct to map blocks for iomap_begin because iomap is not generated by bindgen
+#[repr(C)]
+pub struct ErofsMapBlocks {
+    buf: ErofsBuf,
+    pub(crate) m_pa: u64,
+    pub(crate) m_la: u64,
+    pub(crate) m_plen: u64,
+    pub(crate) m_llen: u64,
+    pub(crate) m_deviceid: u16,
+    pub(crate) m_flags: u32,
+}
+/// Exported as a replacement for erofs_map_blocks.
+#[no_mangle]
+pub unsafe extern "C" fn erofs_map_blocks_rust(
+    k_inode: NonNull<inode>,
+    mut map: NonNull<ErofsMapBlocks>,
+) -> c_int {
+    // SAFETY: super_block and superblockinfo is always initialized in k_inode.
+    let sbi = erofs_sbi(unsafe { NonNull::new(k_inode.as_ref().i_sb).unwrap() });
+    // SAFETY: We are sure that the inode is a Kernel Inode since alloc_inode is called
+    let erofs_inode = unsafe { &*container_of!(k_inode.as_ptr(), KernelInode, k_inode) };
+    // SAFETY: The map is always initialized in the caller.
+    match sbi
+        .filesystem
+        .map(erofs_inode, unsafe { map.as_ref().m_la } as Off)
+    {
+        Ok(m) => unsafe {
+            map.as_mut().m_pa = m.physical.start;
+            map.as_mut().m_la = map.as_ref().m_la;
+            map.as_mut().m_plen = m.physical.len;
+            map.as_mut().m_llen = m.physical.len;
+            map.as_mut().m_deviceid = m.device_id;
+            map.as_mut().m_flags = m.map_type.into();
+            0
+        },
+        Err(e) => i32::from(e) as c_int,
+    }
+}
diff --git a/fs/erofs/rust_bindings.h b/fs/erofs/rust_bindings.h
index 8b71d65e2c0b..ad9aa75a7a2c 100644
--- a/fs/erofs/rust_bindings.h
+++ b/fs/erofs/rust_bindings.h
@@ -24,4 +24,8 @@ extern struct dentry *erofs_lookup_rust(struct inode *inode, struct dentry *dent
 			      unsigned int flags);
 extern struct dentry *erofs_get_parent_rust(struct dentry *dentry);
 extern int erofs_readdir_rust(struct file *file, struct dir_context *ctx);
+
+struct erofs_map_blocks;
+extern int erofs_map_blocks_rust(struct inode *inode,
+				 struct erofs_map_blocks *map);
 #endif
-- 
2.46.0