[RFC PATCH 2/5] rust: block: introduce `kernel::block::bio` module

[Andreas Hindborg <nmi@metaspace.dk>]

From: Andreas Hindborg <a.hindborg@samsung.com>

Add abstractions for working with `struct bio`.

Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
---
 rust/kernel/block.rs            |   1 +
 rust/kernel/block/bio.rs        | 112 +++++++++++++
 rust/kernel/block/bio/vec.rs    | 279 ++++++++++++++++++++++++++++++++
 rust/kernel/block/mq/request.rs |  22 +++
 4 files changed, 414 insertions(+)
 create mode 100644 rust/kernel/block/bio.rs
 create mode 100644 rust/kernel/block/bio/vec.rs

diff --git a/rust/kernel/block.rs b/rust/kernel/block.rs
index 4c93317a568a..1797859551fd 100644
--- a/rust/kernel/block.rs
+++ b/rust/kernel/block.rs
@@ -2,4 +2,5 @@
 
 //! Types for working with the block layer
 
+pub mod bio;
 pub mod mq;
diff --git a/rust/kernel/block/bio.rs b/rust/kernel/block/bio.rs
new file mode 100644
index 000000000000..0d4336cbe9c1
--- /dev/null
+++ b/rust/kernel/block/bio.rs
@@ -0,0 +1,112 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Types for working with the bio layer.
+//!
+//! C header: [`include/linux/blk_types.h`](../../include/linux/blk_types.h)
+
+use core::fmt;
+use core::ptr::NonNull;
+
+mod vec;
+
+pub use vec::BioSegmentIterator;
+pub use vec::Segment;
+
+use crate::types::Opaque;
+
+/// A block device IO descriptor (`struct bio`)
+///
+/// # Invariants
+///
+/// Instances of this type is always reference counted. A call to
+/// `bindings::bio_get()` ensures that the instance is valid for read at least
+/// until a matching call to `bindings :bio_put()`.
+#[repr(transparent)]
+pub struct Bio(Opaque<bindings::bio>);
+
+impl Bio {
+    /// Returns an iterator over segments in this `Bio`. Does not consider
+    /// segments of other bios in this bio chain.
+    #[inline(always)]
+    pub fn segment_iter(&self) -> BioSegmentIterator<'_> {
+        BioSegmentIterator::new(self)
+    }
+
+    /// Get slice referencing the `bio_vec` array of this bio
+    #[inline(always)]
+    fn io_vec(&self) -> &[bindings::bio_vec] {
+        let this = self.0.get();
+
+        // SAFETY: By the type invariant of `Bio` and existence of `&self`,
+        // `this` is valid for read.
+        let io_vec = unsafe { (*this).bi_io_vec };
+
+        // SAFETY: By the type invariant of `Bio` and existence of `&self`,
+        // `this` is valid for read.
+        let length = unsafe { (*this).bi_vcnt };
+
+        // SAFETY: By C API contract, `io_vec` points to `length` consecutive
+        // and properly initialized `bio_vec` values. The array is properly
+        // aligned because it is #[repr(C)]. By C API contract and safety
+        // requirement of `from_raw()`, the elements of the `io_vec` array are
+        // not modified for the duration of the lifetime of `&self`
+        unsafe { core::slice::from_raw_parts(io_vec, length as usize) }
+    }
+
+    /// Return a copy of the `bvec_iter` for this `Bio`. This iterator always
+    /// indexes to a valid `bio_vec` entry.
+    #[inline(always)]
+    fn raw_iter(&self) -> bindings::bvec_iter {
+        // SAFETY: By the type invariant of `Bio` and existence of `&self`,
+        // `self` is valid for read.
+        unsafe { (*self.0.get()).bi_iter }
+    }
+
+    /// Get the next `Bio` in the chain
+    #[inline(always)]
+    fn next(&self) -> Option<&Self> {
+        // SAFETY: By the type invariant of `Bio` and existence of `&self`,
+        // `self` is valid for read.
+        let next = unsafe { (*self.0.get()).bi_next };
+        // SAFETY: By C API contract `bi_next` has nonzero reference count if it
+        // is not null, for at least the duration of the lifetime of &self.
+        unsafe { Self::from_raw(next) }
+    }
+
+    /// Create an instance of `Bio` from a raw pointer.
+    ///
+    /// # Safety
+    ///
+    /// If `ptr` is not null, caller must ensure positive refcount for the
+    /// pointee and immutability for the duration of the returned lifetime.
+    #[inline(always)]
+    pub(crate) unsafe fn from_raw<'a>(ptr: *mut bindings::bio) -> Option<&'a Self> {
+        Some(
+            // SAFETY: by the safety requirement of this funciton, `ptr` is
+            // valid for read for the duration of the returned lifetime
+            unsafe { &*NonNull::new(ptr)?.as_ptr().cast::<Bio>() },
+        )
+    }
+}
+
+impl core::fmt::Display for Bio {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Bio({:?})", self.0.get())
+    }
+}
+
+/// An iterator over `Bio`
+pub struct BioIterator<'a> {
+    pub(crate) bio: Option<&'a Bio>,
+}
+
+impl<'a> core::iter::Iterator for BioIterator<'a> {
+    type Item = &'a Bio;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<&'a Bio> {
+        let current = self.bio.take()?;
+        self.bio = current.next();
+        Some(current)
+    }
+}
diff --git a/rust/kernel/block/bio/vec.rs b/rust/kernel/block/bio/vec.rs
new file mode 100644
index 000000000000..b61380807f38
--- /dev/null
+++ b/rust/kernel/block/bio/vec.rs
@@ -0,0 +1,279 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Types for working with `struct bio_vec` IO vectors
+//!
+//! C header: [`include/linux/bvec.h`](../../include/linux/bvec.h)
+
+use super::Bio;
+use crate::error::Result;
+use crate::folio::UniqueFolio;
+use crate::page::Page;
+use core::fmt;
+use core::mem::ManuallyDrop;
+
+/// A wrapper around a `strutct bio_vec` - a contiguous range of physical memory addresses
+///
+/// # Invariants
+///
+/// `bio_vec` must always be initialized and valid for read and write
+pub struct Segment<'a> {
+    bio_vec: bindings::bio_vec,
+    _marker: core::marker::PhantomData<&'a ()>,
+}
+
+impl Segment<'_> {
+    /// Get he lenght of the segment in bytes
+    #[inline(always)]
+    pub fn len(&self) -> usize {
+        self.bio_vec.bv_len as usize
+    }
+
+    /// Returns true if the length of the segment is 0
+    #[inline(always)]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Get the offset field of the `bio_vec`
+    #[inline(always)]
+    pub fn offset(&self) -> usize {
+        self.bio_vec.bv_offset as usize
+    }
+
+    /// Copy data of this segment into `folio`.
+    ///
+    /// Note: Disregards `self.offset()`
+    #[inline(always)]
+    pub fn copy_to_folio(&self, dst_folio: &mut UniqueFolio) -> Result {
+        // SAFETY: self.bio_vec is valid and thus bv_page must be a valid
+        // pointer to a `struct page`. We do not own the page, but we prevent
+        // drop by wrapping the `Page` in `ManuallyDrop`.
+        let src_page = ManuallyDrop::new(unsafe { Page::from_raw(self.bio_vec.bv_page) });
+
+        src_page.with_slice_into_page(|src| {
+            dst_folio.with_slice_into_page_mut(0, |dst| {
+                dst.copy_from_slice(src);
+                Ok(())
+            })
+        })
+    }
+
+    /// Copy data to the page of this segment from `src`.
+    ///
+    /// Note: Disregards `self.offset()`
+    pub fn copy_from_folio(&mut self, src_folio: &UniqueFolio) -> Result {
+        // SAFETY: self.bio_vec is valid and thus bv_page must be a valid
+        // pointer to a `struct page`. We do not own the page, but we prevent
+        // drop by wrapping the `Page` in `ManuallyDrop`.
+        let mut dst_page = ManuallyDrop::new(unsafe { Page::from_raw(self.bio_vec.bv_page) });
+
+        dst_page.with_slice_into_page_mut(|dst| {
+            src_folio.with_slice_into_page(0, |src| {
+                dst.copy_from_slice(src);
+                Ok(())
+            })
+        })
+    }
+}
+
+impl core::fmt::Display for Segment<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "Segment {:?} len: {}",
+            self.bio_vec.bv_page, self.bio_vec.bv_len
+        )
+    }
+}
+
+/// An iterator over `Segment`
+///
+/// # Invariants
+///
+/// If `iter.bi_size` > 0, `iter` must always index a valid `bio_vec` in `bio.io_vec()`.
+pub struct BioSegmentIterator<'a> {
+    bio: &'a Bio,
+    iter: bindings::bvec_iter,
+}
+
+impl<'a> BioSegmentIterator<'a> {
+    /// Creeate a new segemnt iterator for iterating the segments of `bio`. The
+    /// iterator starts at the beginning of `bio`.
+    #[inline(always)]
+    pub(crate) fn new(bio: &'a Bio) -> BioSegmentIterator<'_> {
+        // SAFETY: `bio.raw_iter()` returns an index that indexes into a valid
+        // `bio_vec` in `bio.io_vec()`.
+        Self {
+            bio,
+            iter: bio.raw_iter(),
+        }
+    }
+
+    // The accessors in this implementation block are modelled after C side
+    // macros and static functions `bvec_iter_*` and `mp_bvec_iter_*` from
+    // bvec.h.
+
+    /// Construct a `bio_vec` from the current iterator state.
+    ///
+    /// This will return a `bio_vec`of size <= PAGE_SIZE
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    unsafe fn io_vec(&self) -> bindings::bio_vec {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        unsafe {
+            bindings::bio_vec {
+                bv_page: self.page(),
+                bv_len: self.len(),
+                bv_offset: self.offset(),
+            }
+        }
+    }
+
+    /// Get the currently indexed `bio_vec` entry.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn bvec(&self) -> &bindings::bio_vec {
+        // SAFETY: By the safety requirement of this function and the type
+        // invariant of `Self`, `self.iter.bi_idx` indexes into a valid
+        // `bio_vec`
+        unsafe { self.bio.io_vec().get_unchecked(self.iter.bi_idx as usize) }
+    }
+
+    /// Get the currently indexed page, indexing into pages of order > 0.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn page(&self) -> *mut bindings::page {
+        // SAFETY: By C API contract, the following offset cannot exceed pages
+        // allocated to this bio.
+        unsafe { self.mp_page().add(self.mp_page_idx()) }
+    }
+
+    /// Get the remaining bytes in the current page. Never more than PAGE_SIZE.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn len(&self) -> u32 {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        unsafe { self.mp_len().min((bindings::PAGE_SIZE as u32) - self.offset()) }
+    }
+
+    /// Get the offset from the last page boundary in the currently indexed
+    /// `bio_vec` entry. Never more than PAGE_SIZE.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn offset(&self) -> u32 {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        unsafe { self.mp_offset() % (bindings::PAGE_SIZE as u32) }
+    }
+
+    /// Return the first page of the currently indexed `bio_vec` entry. This
+    /// might be a multi-page entry, meaning that page might have order > 0.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn mp_page(&self) -> *mut bindings::page {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        unsafe { self.bvec().bv_page }
+    }
+
+    /// Get the offset in whole pages into the currently indexed `bio_vec`. This
+    /// can be more than 0 is the page has order > 0.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn mp_page_idx(&self) -> usize {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        (unsafe { self.mp_offset() } / (bindings::PAGE_SIZE as u32)) as usize
+    }
+
+    /// Get the offset in the currently indexed `bio_vec` multi-page entry. This
+    /// can be more than `PAGE_SIZE` if the page has order > 0.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn mp_offset(&self) -> u32 {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        unsafe { self.bvec().bv_offset + self.iter.bi_bvec_done }
+    }
+
+    /// Get the number of remaining bytes for the currently indexed `bio_vec`
+    /// entry. Can be more than PAGE_SIZE for `bio_vec` entries with pages of
+    /// order > 0.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `self.iter.bi_size` > 0 before calling this
+    /// method.
+    #[inline(always)]
+    unsafe fn mp_len(&self) -> u32 {
+        // SAFETY: By safety requirement of this function `self.iter.bi_size` is
+        // greater than 0.
+        self.iter
+            .bi_size
+            .min(unsafe { self.bvec().bv_len } - self.iter.bi_bvec_done)
+    }
+}
+
+impl<'a> core::iter::Iterator for BioSegmentIterator<'a> {
+    type Item = Segment<'a>;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.iter.bi_size == 0 {
+            return None;
+        }
+
+        // SAFETY: We checked that `self.iter.bi_size` > 0 above.
+        let bio_vec_ret = unsafe { self.io_vec() };
+
+        // SAFETY: By existence of reference `&bio`, `bio.0` contains a valid
+        // `struct bio`. By type invariant of `BioSegmentItarator` `self.iter`
+        // indexes into a valid `bio_vec` entry. By C API contracit, `bv_len`
+        // does not exceed the size of the bio.
+        unsafe {
+            bindings::bio_advance_iter_single(
+                self.bio.0.get(),
+                &mut self.iter as *mut bindings::bvec_iter,
+                bio_vec_ret.bv_len,
+            )
+        };
+
+        Some(Segment {
+            bio_vec: bio_vec_ret,
+            _marker: core::marker::PhantomData,
+        })
+    }
+}
diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
index b4dacac5e091..cccffde45981 100644
--- a/rust/kernel/block/mq/request.rs
+++ b/rust/kernel/block/mq/request.rs
@@ -12,6 +12,9 @@
 };
 use core::{ffi::c_void, marker::PhantomData, ops::Deref};
 
+use crate::block::bio::Bio;
+use crate::block::bio::BioIterator;
+
 /// A wrapper around a blk-mq `struct request`. This represents an IO request.
 ///
 /// # Invariants
@@ -84,6 +87,25 @@ pub fn complete(&self) {
         }
     }
 
+    /// Get a wrapper for the first Bio in this request
+    #[inline(always)]
+    pub fn bio(&self) -> Option<&Bio> {
+        // SAFETY: By type invariant of `Self`, `self.0` is valid and the deref
+        // is safe.
+        let ptr = unsafe { (*self.0.get()).bio };
+        // SAFETY: By C API contract, if `bio` is not null it will have a
+        // positive refcount at least for the duration of the lifetime of
+        // `&self`.
+        unsafe { Bio::from_raw(ptr) }
+    }
+
+    /// Get an iterator over all bio structurs in this request
+    #[inline(always)]
+    pub fn bio_iter(&self) -> BioIterator<'_> {
+        BioIterator { bio: self.bio() }
+    }
+
+    // TODO: Check if inline is still required for cross language LTO inlining into module
     /// Get the target sector for the request
     #[inline(always)]
     pub fn sector(&self) -> usize {
-- 
2.44.0