[PATCH v2] rust: file: optimize rust symbol generation for FileDescriptorReservation

[Kunwu Chan <kunwu.chan@linux.dev>]

From: Kunwu Chan <kunwu.chan@hotmail.com>

When build the kernel using the llvm-18.1.3-rust-1.85.0-x86_64
with ARCH=arm64, the following symbols are generated:

$ nm vmlinux | grep ' _R'.*FileDescriptorReservation | rustfilt
... T <kernel::fs::file::FileDescriptorReservation>::fd_install
... T <kernel::fs::file::FileDescriptorReservation>::get_unused_fd_flags
... T <kernel::fs::file::FileDescriptorReservation as core::ops::drop::Drop>::drop

These Rust symbols are trivial wrappers around the C functions
fd_install, put_unused_fd and put_task_struct. It
doesn't make sense to go through a trivial wrapper for these
functions, so mark them inline.

Link: https://github.com/Rust-for-Linux/linux/issues/1145
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Co-developed-by: Grace Deng <Grace.Deng006@Gmail.com>
Signed-off-by: Grace Deng <Grace.Deng006@Gmail.com>
Signed-off-by: Kunwu Chan <kunwu.chan@hotmail.com>

---
Changes in v2:
 - Add link and Suggested-by
 - Mark 'reserved_fd' as inline
 - Reword commit msg
---
 rust/kernel/fs/file.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index e03dbe14d62a..736209a1b983 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -392,6 +392,7 @@ pub struct FileDescriptorReservation {
 
 impl FileDescriptorReservation {
     /// Creates a new file descriptor reservation.
+    #[inline]
     pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
         // SAFETY: FFI call, there are no safety requirements on `flags`.
         let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
@@ -405,6 +406,7 @@ pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
     }
 
     /// Returns the file descriptor number that was reserved.
+    #[inline]
     pub fn reserved_fd(&self) -> u32 {
         self.fd
     }
@@ -413,6 +415,7 @@ pub fn reserved_fd(&self) -> u32 {
     ///
     /// The previously reserved file descriptor is bound to `file`. This method consumes the
     /// [`FileDescriptorReservation`], so it will not be usable after this call.
+    #[inline]
     pub fn fd_install(self, file: ARef<File>) {
         // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`. We have not yet used
         // the fd, so it is still valid, and `current` still refers to the same task, as this type
@@ -433,6 +436,7 @@ pub fn fd_install(self, file: ARef<File>) {
 }
 
 impl Drop for FileDescriptorReservation {
+    #[inline]
     fn drop(&mut self) {
         // SAFETY: By the type invariants of this type, `self.fd` was previously returned by
         // `get_unused_fd_flags`. We have not yet used the fd, so it is still valid, and `current`
-- 
2.43.0