rust: folio: introduce basic support for folios

We implement the minimum needed in `read_folio` (part of
`struct address_space_operations`) to implement simple read-only file
systems.

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

Author: wedsonaf

rust/bindings/bindings_helper.h

@@ -11,6 +11,7 @@
 #include <linux/fs.h>
 #include <linux/fs_context.h>
 #include <linux/slab.h>
+#include <linux/pagemap.h>
 #include <linux/refcount.h>
 #include <linux/wait.h>
 #include <linux/sched.h>
@@ -26,3 +27,5 @@ const slab_flags_t BINDINGS_SLAB_MEM_SPREAD = SLAB_MEM_SPREAD;
 const unsigned long BINDINGS_SB_RDONLY = SB_RDONLY;
 
 const loff_t BINDINGS_MAX_LFS_FILESIZE = MAX_LFS_FILESIZE;
+
+const size_t BINDINGS_PAGE_SIZE = PAGE_SIZE;

rust/bindings/lib.rs

@@ -58,3 +58,5 @@ pub const SLAB_MEM_SPREAD: slab_flags_t = BINDINGS_SLAB_MEM_SPREAD;
 pub const SB_RDONLY: core::ffi::c_ulong = BINDINGS_SB_RDONLY;
 
 pub const MAX_LFS_FILESIZE: loff_t = BINDINGS_MAX_LFS_FILESIZE;
+
+pub const PAGE_SIZE: usize = BINDINGS_PAGE_SIZE;

rust/helpers.c

@@ -23,10 +23,14 @@
 #include <kunit/test-bug.h>
 #include <linux/bug.h>
 #include <linux/build_bug.h>
+#include <linux/cacheflush.h>
 #include <linux/err.h>
 #include <linux/errname.h>
 #include <linux/fs.h>
+#include <linux/highmem.h>
+#include <linux/mm.h>
 #include <linux/mutex.h>
+#include <linux/pagemap.h>
 #include <linux/refcount.h>
 #include <linux/sched/signal.h>
 #include <linux/spinlock.h>
@@ -145,6 +149,60 @@ struct kunit *rust_helper_kunit_get_current_test(void)
 }
 EXPORT_SYMBOL_GPL(rust_helper_kunit_get_current_test);
 
+void rust_helper_folio_get(struct folio *folio)
+{
+	folio_get(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_folio_get);
+
+void rust_helper_folio_put(struct folio *folio)
+{
+	folio_put(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_folio_put);
+
+loff_t rust_helper_folio_pos(struct folio *folio)
+{
+	return folio_pos(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_folio_pos);
+
+size_t rust_helper_folio_size(struct folio *folio)
+{
+	return folio_size(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_folio_size);
+
+void rust_helper_folio_mark_uptodate(struct folio *folio)
+{
+	folio_mark_uptodate(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_folio_mark_uptodate);
+
+void rust_helper_folio_set_error(struct folio *folio)
+{
+	folio_set_error(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_folio_set_error);
+
+void rust_helper_flush_dcache_folio(struct folio *folio)
+{
+	flush_dcache_folio(folio);
+}
+EXPORT_SYMBOL_GPL(rust_helper_flush_dcache_folio);
+
+void *rust_helper_kmap_local_folio(struct folio *folio, size_t offset)
+{
+	return kmap_local_folio(folio, offset);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kmap_local_folio);
+
+void rust_helper_kunmap_local(const void *vaddr)
+{
+	kunmap_local(vaddr);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kunmap_local);
+
 void rust_helper_i_uid_write(struct inode *inode, uid_t uid)
 {
 	i_uid_write(inode, uid);
@@ -163,6 +221,12 @@ off_t rust_helper_i_size_read(const struct inode *inode)
 }
 EXPORT_SYMBOL_GPL(rust_helper_i_size_read);
 
+void rust_helper_mapping_set_large_folios(struct address_space *mapping)
+{
+	mapping_set_large_folios(mapping);
+}
+EXPORT_SYMBOL_GPL(rust_helper_mapping_set_large_folios);
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.

rust/kernel/folio.rs

@@ -0,0 +1,153 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Groups of contiguous pages, folios.
+//!
+//! C headers: [`include/linux/mm.h`](../../include/linux/mm.h)
+
+use crate::error::{code::*, Result};
+use crate::types::{AlwaysRefCounted, Opaque, ScopeGuard};
+use core::{cmp::min, ptr};
+
+/// Wraps the kernel's `struct folio`.
+///
+/// # Invariants
+///
+/// Instances of this type are always ref-counted, that is, a call to `folio_get` ensures that the
+/// allocation remains valid at least until the matching call to `folio_put`.
+#[repr(transparent)]
+pub struct Folio(Opaque<bindings::folio>);
+
+// SAFETY: The type invariants guarantee that `Folio` is always ref-counted.
+unsafe impl AlwaysRefCounted for Folio {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+        unsafe { bindings::folio_get(self.0.get()) };
+    }
+
+    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
+        unsafe { bindings::folio_put(obj.cast().as_ptr()) }
+    }
+}
+
+impl Folio {
+    /// Returns the byte position of this folio in its file.
+    pub fn pos(&self) -> i64 {
+        // SAFETY: The folio is valid because the shared reference implies a non-zero refcount.
+        unsafe { bindings::folio_pos(self.0.get()) }
+    }
+
+    /// Returns the byte size of this folio.
+    pub fn size(&self) -> usize {
+        // SAFETY: The folio is valid because the shared reference implies a non-zero refcount.
+        unsafe { bindings::folio_size(self.0.get()) }
+    }
+
+    /// Flushes the data cache for the pages that make up the folio.
+    pub fn flush_dcache(&self) {
+        // SAFETY: The folio is valid because the shared reference implies a non-zero refcount.
+        unsafe { bindings::flush_dcache_folio(self.0.get()) }
+    }
+}
+
+/// A locked [`Folio`].
+pub struct LockedFolio<'a>(&'a Folio);
+
+impl LockedFolio<'_> {
+    /// Creates a new locked folio from a raw pointer.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that the folio is valid and locked. Additionally, that the
+    /// responsibility of unlocking is transferred to the new instance of [`LockedFolio`]. Lastly,
+    /// that the returned [`LockedFolio`] doesn't outlive the refcount that keeps it alive.
+    #[allow(dead_code)]
+    pub(crate) unsafe fn from_raw(folio: *const bindings::folio) -> Self {
+        let ptr = folio.cast();
+        // SAFETY: The safety requirements ensure that `folio` (from which `ptr` is derived) is
+        // valid and will remain valid while the `LockedFolio` instance lives.
+        Self(unsafe { &*ptr })
+    }
+
+    /// Marks the folio as being up to date.
+    pub fn mark_uptodate(&mut self) {
+        // SAFETY: The folio is valid because the shared reference implies a non-zero refcount.
+        unsafe { bindings::folio_mark_uptodate(self.0 .0.get()) }
+    }
+
+    /// Sets the error flag on the folio.
+    pub fn set_error(&mut self) {
+        // SAFETY: The folio is valid because the shared reference implies a non-zero refcount.
+        unsafe { bindings::folio_set_error(self.0 .0.get()) }
+    }
+
+    fn for_each_page(
+        &mut self,
+        offset: usize,
+        len: usize,
+        mut cb: impl FnMut(&mut [u8]) -> Result,
+    ) -> Result {
+        let mut remaining = len;
+        let mut next_offset = offset;
+
+        // Check that we don't overflow the folio.
+        let end = offset.checked_add(len).ok_or(EDOM)?;
+        if end > self.size() {
+            return Err(EINVAL);
+        }
+
+        while remaining > 0 {
+            let page_offset = next_offset & (bindings::PAGE_SIZE - 1);
+            let usable = min(remaining, bindings::PAGE_SIZE - page_offset);
+            // SAFETY: The folio is valid because the shared reference implies a non-zero refcount;
+            // `next_offset` is also guaranteed be lesss than the folio size.
+            let ptr = unsafe { bindings::kmap_local_folio(self.0 .0.get(), next_offset) };
+
+            // SAFETY: `ptr` was just returned by the `kmap_local_folio` above.
+            let _guard = ScopeGuard::new(|| unsafe { bindings::kunmap_local(ptr) });
+
+            // SAFETY: `kmap_local_folio` maps whole page so we know it's mapped for at least
+            // `usable` bytes.
+            let s = unsafe { core::slice::from_raw_parts_mut(ptr.cast::<u8>(), usable) };
+            cb(s)?;
+
+            next_offset += usable;
+            remaining -= usable;
+        }
+
+        Ok(())
+    }
+
+    /// Writes the given slice into the folio.
+    pub fn write(&mut self, offset: usize, data: &[u8]) -> Result {
+        let mut remaining = data;
+
+        self.for_each_page(offset, data.len(), |s| {
+            s.copy_from_slice(&remaining[..s.len()]);
+            remaining = &remaining[s.len()..];
+            Ok(())
+        })
+    }
+
+    /// Writes zeroes into the folio.
+    pub fn zero_out(&mut self, offset: usize, len: usize) -> Result {
+        self.for_each_page(offset, len, |s| {
+            s.fill(0);
+            Ok(())
+        })
+    }
+}
+
+impl core::ops::Deref for LockedFolio<'_> {
+    type Target = Folio;
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl Drop for LockedFolio<'_> {
+    fn drop(&mut self) {
+        // SAFETY: The folio is valid because the shared reference implies a non-zero refcount.
+        unsafe { bindings::folio_unlock(self.0 .0.get()) }
+    }
+}

rust/kernel/lib.rs

@@ -34,6 +34,7 @@ extern crate self as kernel;
 mod allocator;
 mod build_assert;
 pub mod error;
+pub mod folio;
 pub mod fs;
 pub mod init;
 pub mod ioctl;