rust: fs: allow per-inode data

This allows file systems to attach extra [typed] data to each inode. If
no data is needed, we use the regular inode kmem_cache, otherwise we
create a new one.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>

Author: wedsonaf

rust/helpers.c

@@ -205,6 +205,13 @@ void rust_helper_kunmap_local(const void *vaddr)
 }
 EXPORT_SYMBOL_GPL(rust_helper_kunmap_local);
 
+void *rust_helper_alloc_inode_sb(struct super_block *sb,
+				 struct kmem_cache *cache, gfp_t gfp)
+{
+	return alloc_inode_sb(sb, cache, gfp);
+}
+EXPORT_SYMBOL_GPL(rust_helper_alloc_inode_sb);
+
 void rust_helper_i_uid_write(struct inode *inode, uid_t uid)
 {
 	i_uid_write(inode, uid);

rust/kernel/fs.rs

@@ -16,10 +16,11 @@ pub mod ro {
     use crate::error::{code::*, from_result, to_result, Error, Result};
     use crate::types::{ARef, AlwaysRefCounted, Either, ForeignOwnable, Opaque};
     use crate::{
-        bindings, folio::LockedFolio, init::PinInit, str::CStr, time::Time, try_pin_init,
-        ThisModule,
+        bindings, container_of, folio::LockedFolio, init::PinInit, mem_cache::MemCache, str::CStr,
+        time::Time, try_pin_init, ThisModule,
     };
-    use core::{marker::PhantomData, marker::PhantomPinned, mem::ManuallyDrop, pin::Pin, ptr};
+    use core::mem::{size_of, ManuallyDrop, MaybeUninit};
+    use core::{marker::PhantomData, marker::PhantomPinned, pin::Pin, ptr};
     use macros::{pin_data, pinned_drop};
 
     /// Type of superblock keying.
@@ -38,6 +39,9 @@ pub mod ro {
         /// Data associated with each file system instance (super-block).
         type Data: ForeignOwnable + Send + Sync;
 
+        /// Type of data associated with each inode.
+        type INodeData: Send + Sync;
+
         /// The name of the file system type.
         const NAME: &'static CStr;
 
@@ -173,6 +177,7 @@ pub mod ro {
     pub struct Registration {
         #[pin]
         fs: Opaque<bindings::file_system_type>,
+        inode_cache: Option<MemCache>,
         #[pin]
         _pin: PhantomPinned,
     }
@@ -190,6 +195,14 @@ pub mod ro {
         pub fn new<T: Type + ?Sized>(module: &'static ThisModule) -> impl PinInit<Self, Error> {
             try_pin_init!(Self {
                 _pin: PhantomPinned,
+                inode_cache: if size_of::<T::INodeData>() == 0 {
+                    None
+                } else {
+                    Some(MemCache::try_new::<INodeWithData<T::INodeData>>(
+                        T::NAME,
+                        Some(Self::inode_init_once_callback::<T>),
+                    )?)
+                },
                 fs <- Opaque::try_ffi_init(|fs_ptr| {
                     // SAFETY: `pin_init_from_closure` guarantees that `fs_ptr` is valid for write.
                     let fs = unsafe { &mut *fs_ptr };
@@ -244,6 +257,16 @@ pub mod ro {
                 unsafe { T::Data::from_foreign(ptr) };
             }
         }
+
+        unsafe extern "C" fn inode_init_once_callback<T: Type + ?Sized>(
+            outer_inode: *mut core::ffi::c_void,
+        ) {
+            let ptr = outer_inode.cast::<INodeWithData<T::INodeData>>();
+
+            // SAFETY: This is only used in `new`, so we know that we have a valid `INodeWithData`
+            // instance whose inode part can be initialised.
+            unsafe { bindings::inode_init_once(ptr::addr_of_mut!((*ptr).inode)) };
+        }
     }
 
     #[pinned_drop]
@@ -280,6 +303,15 @@ pub mod ro {
             unsafe { &*(*self.0.get()).i_sb.cast() }
         }
 
+        /// Returns the data associated with the inode.
+        pub fn data(&self) -> &T::INodeData {
+            let outerp = container_of!(self.0.get(), INodeWithData<T::INodeData>, inode);
+            // SAFETY: `self` is guaranteed to be valid by the existence of a shared reference
+            // (`&self`) to it. Additionally, we know `T::INodeData` is always initialised in an
+            // `INode`.
+            unsafe { &*(*outerp).data.as_ptr() }
+        }
+
         /// Returns the size of the inode contents.
         pub fn size(&self) -> i64 {
             // SAFETY: `self` is guaranteed to be valid by the existence of a shared reference.
@@ -300,15 +332,29 @@ pub mod ro {
         }
     }
 
+    struct INodeWithData<T> {
+        data: MaybeUninit<T>,
+        inode: bindings::inode,
+    }
+
     /// An inode that is locked and hasn't been initialised yet.
     #[repr(transparent)]
     pub struct NewINode<T: Type + ?Sized>(ARef<INode<T>>);
 
     impl<T: Type + ?Sized> NewINode<T> {
         /// Initialises the new inode with the given parameters.
-        pub fn init(self, params: INodeParams) -> Result<ARef<INode<T>>> {
-            // SAFETY: This is a new inode, so it's safe to manipulate it mutably.
-            let inode = unsafe { &mut *self.0 .0.get() };
+        pub fn init(self, params: INodeParams<T::INodeData>) -> Result<ARef<INode<T>>> {
+            let outerp = container_of!(self.0 .0.get(), INodeWithData<T::INodeData>, inode);
+
+            // SAFETY: This is a newly-created inode. No other references to it exist, so it is
+            // safe to mutably dereference it.
+            let outer = unsafe { &mut *outerp.cast_mut() };
+
+            // N.B. We must always write this to a newly allocated inode because the free callback
+            // expects the data to be initialised and drops it.
+            outer.data.write(params.value);
+
+            let inode = &mut outer.inode;
 
             let mode = match params.typ {
                 INodeType::Dir => {
@@ -425,7 +471,7 @@ pub mod ro {
     /// Required inode parameters.
     ///
     /// This is used when creating new inodes.
-    pub struct INodeParams {
+    pub struct INodeParams<T> {
         /// The access mode. It's a mask that grants execute (1), write (2) and read (4) access to
         /// everyone, the owner group, and the owner.
         pub mode: u16,
@@ -460,6 +506,9 @@ pub mod ro {
 
         /// Last access time.
         pub atime: Time,
+
+        /// Value to attach to this node.
+        pub value: T,
     }
 
     /// A file system super block.
@@ -759,8 +808,12 @@ pub mod ro {
         }
 
         const SUPER_BLOCK: bindings::super_operations = bindings::super_operations {
-            alloc_inode: None,
-            destroy_inode: None,
+            alloc_inode: if size_of::<T::INodeData>() != 0 {
+                Some(Self::alloc_inode_callback)
+            } else {
+                None
+            },
+            destroy_inode: Some(Self::destroy_inode_callback),
             free_inode: None,
             dirty_inode: None,
             write_inode: None,
@@ -790,6 +843,61 @@ pub mod ro {
             shutdown: None,
         };
 
+        unsafe extern "C" fn alloc_inode_callback(
+            sb: *mut bindings::super_block,
+        ) -> *mut bindings::inode {
+            // SAFETY: The callback contract guarantees that `sb` is valid for read.
+            let super_type = unsafe { (*sb).s_type };
+
+            // SAFETY: This callback is only used in `Registration`, so `super_type` is necessarily
+            // embedded in a `Registration`, which is guaranteed to be valid because it has a
+            // superblock associated to it.
+            let reg = unsafe { &*container_of!(super_type, Registration, fs) };
+
+            // SAFETY: `sb` and `cache` are guaranteed to be valid by the callback contract and by
+            // the existence of a superblock respectively.
+            let ptr = unsafe {
+                bindings::alloc_inode_sb(sb, MemCache::ptr(&reg.inode_cache), bindings::GFP_KERNEL)
+            }
+            .cast::<INodeWithData<T::INodeData>>();
+            if ptr.is_null() {
+                return ptr::null_mut();
+            }
+            ptr::addr_of_mut!((*ptr).inode)
+        }
+
+        unsafe extern "C" fn destroy_inode_callback(inode: *mut bindings::inode) {
+            // SAFETY: By the C contrat, inode is a valid pointer.
+            let is_bad = unsafe { bindings::is_bad_inode(inode) };
+
+            // SAFETY: The inode is guaranteed to be valid by the callback contract. Additionally,
+            // the superblock is also guaranteed to still be valid by the inode existence.
+            let super_type = unsafe { (*(*inode).i_sb).s_type };
+
+            // SAFETY: This callback is only used in `Registration`, so `super_type` is necessarily
+            // embedded in a `Registration`, which is guaranteed to be valid because it has a
+            // superblock associated to it.
+            let reg = unsafe { &*container_of!(super_type, Registration, fs) };
+            let ptr = container_of!(inode, INodeWithData<T::INodeData>, inode).cast_mut();
+
+            if !is_bad {
+                // SAFETY: The code either initialises the data or marks the inode as bad, since
+                // it's not bad, it's safey to drop it here.
+                unsafe { ptr::drop_in_place((*ptr).data.as_mut_ptr()) };
+            }
+
+            if size_of::<T::INodeData>() == 0 {
+                // SAFETY: When the size of `INodeData` is zero, we don't use a separate
+                // mem_cache, so it is allocated from the regular mem_cache, which is what
+                // `free_inode_nonrcu` uses to free the inode.
+                unsafe { bindings::free_inode_nonrcu(inode) };
+            } else {
+                // The callback contract guarantees that the inode was previously allocated via the
+                // `alloc_inode_callback` callback, so it is safe to free it back to the cache.
+                unsafe { bindings::kmem_cache_free(MemCache::ptr(&reg.inode_cache), ptr.cast()) };
+            }
+        }
+
         unsafe extern "C" fn statfs_callback(
             dentry: *mut bindings::dentry,
             buf: *mut bindings::kstatfs,
@@ -1089,6 +1197,7 @@ pub mod ro {
     /// impl fs::ro::Type for MyFs {
     ///     // ...
     /// #    type Data = ();
+    /// #    type INodeData = ();
     /// #    const NAME: &'static CStr = c_str!("myfs");
     /// #    fn fill_super(
     /// #        _: fs::ro::NewSuperBlock<'_, Self>

samples/rust/rust_rofs.rs

@@ -53,6 +53,7 @@ const ENTRIES: [Entry; 4] = [
 struct RoFs;
 impl fs::ro::Type for RoFs {
     type Data = ();
+    type INodeData = &'static Entry;
     const NAME: &'static CStr = c_str!("rust-fs");
 
     fn fill_super(sb: NewSuperBlock<'_, Self>) -> Result<&SuperBlock<Self>> {
@@ -77,6 +78,7 @@ impl fs::ro::Type for RoFs {
                 atime: UNIX_EPOCH,
                 ctime: UNIX_EPOCH,
                 mtime: UNIX_EPOCH,
+                value: &ENTRIES[0],
             })?,
         };
         sb.init_root(root)
@@ -125,6 +127,7 @@ impl fs::ro::Type for RoFs {
                         atime: UNIX_EPOCH,
                         ctime: UNIX_EPOCH,
                         mtime: UNIX_EPOCH,
+                        value: e,
                     }),
                 };
             }
@@ -134,11 +137,7 @@ impl fs::ro::Type for RoFs {
     }
 
     fn read_folio(inode: &INode<Self>, mut folio: LockedFolio<'_>) -> Result {
-        let data = match inode.ino() {
-            2 => ENTRIES[2].contents,
-            3 => ENTRIES[3].contents,
-            _ => return Err(EINVAL),
-        };
+        let data = inode.data().contents;
 
         let pos = usize::try_from(folio.pos()).unwrap_or(usize::MAX);
         let copied = if pos >= data.len() {