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Message-ID: <aHYkkQpFOIcKuK-v@google.com>
Date: Tue, 15 Jul 2025 09:51:13 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>,
Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Jens Axboe <axboe@...nel.dk>,
linux-block@...r.kernel.org, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 13/16] rust: block: add `GenDisk` private data support
On Fri, Jul 11, 2025 at 01:43:14PM +0200, Andreas Hindborg wrote:
> Allow users of the rust block device driver API to install private data in
> the `GenDisk` structure.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
> drivers/block/rnull/rnull.rs | 8 ++++---
> rust/kernel/block/mq.rs | 7 +++---
> rust/kernel/block/mq/gen_disk.rs | 32 ++++++++++++++++++++++----
> rust/kernel/block/mq/operations.rs | 46 ++++++++++++++++++++++++++++++--------
> 4 files changed, 74 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
> index d09bc77861e4..a012c59ecb3c 100644
> --- a/drivers/block/rnull/rnull.rs
> +++ b/drivers/block/rnull/rnull.rs
> @@ -62,14 +62,16 @@ fn new(
> .logical_block_size(block_size)?
> .physical_block_size(block_size)?
> .rotational(rotational)
> - .build(fmt!("{}", name.to_str()?), tagset)
> + .build(fmt!("{}", name.to_str()?), tagset, ())
> }
> }
>
> #[vtable]
> impl Operations for NullBlkDevice {
> + type QueueData = ();
> +
> #[inline(always)]
> - fn queue_rq(rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
> + fn queue_rq(_queue_data: (), rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
> mq::Request::end_ok(rq)
> .map_err(|_e| kernel::error::code::EIO)
> // We take no refcounts on the request, so we expect to be able to
> @@ -80,5 +82,5 @@ fn queue_rq(rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
> Ok(())
> }
>
> - fn commit_rqs() {}
> + fn commit_rqs(_queue_data: ()) {}
> }
> diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
> index faa3ccb5a49a..34b7425fa94d 100644
> --- a/rust/kernel/block/mq.rs
> +++ b/rust/kernel/block/mq.rs
> @@ -69,20 +69,21 @@
> //!
> //! #[vtable]
> //! impl Operations for MyBlkDevice {
> +//! type QueueData = ();
> //!
> -//! fn queue_rq(rq: ARef<Request<Self>>, _is_last: bool) -> Result {
> +//! fn queue_rq(_queue_data: (), rq: ARef<Request<Self>>, _is_last: bool) -> Result {
> //! Request::end_ok(rq);
> //! Ok(())
> //! }
> //!
> -//! fn commit_rqs() {}
> +//! fn commit_rqs(_queue_data: ()) {}
> //! }
> //!
> //! let tagset: Arc<TagSet<MyBlkDevice>> =
> //! Arc::pin_init(TagSet::new(1, 256, 1), flags::GFP_KERNEL)?;
> //! let mut disk = gen_disk::GenDiskBuilder::new()
> //! .capacity_sectors(4096)
> -//! .build(format_args!("myblk"), tagset)?;
> +//! .build(format_args!("myblk"), tagset, ())?;
> //!
> //! # Ok::<(), kernel::error::Error>(())
> //! ```
> diff --git a/rust/kernel/block/mq/gen_disk.rs b/rust/kernel/block/mq/gen_disk.rs
> index 7ab049ec591b..954bc64cfa59 100644
> --- a/rust/kernel/block/mq/gen_disk.rs
> +++ b/rust/kernel/block/mq/gen_disk.rs
> @@ -12,6 +12,7 @@
> static_lock_class,
> str::NullTerminatedFormatter,
> sync::Arc,
> + types::{ForeignOwnable, ScopeGuard},
> };
> use core::fmt::{self, Write};
>
> @@ -97,7 +98,14 @@ pub fn build<T: Operations>(
> self,
> name: fmt::Arguments<'_>,
> tagset: Arc<TagSet<T>>,
> + queue_data: T::QueueData,
> ) -> Result<GenDisk<T>> {
> + let data = queue_data.into_foreign();
> + let recover_data = ScopeGuard::new(|| {
> + // SAFETY: T::QueueData was created by the call to `into_foreign()` above
> + unsafe { T::QueueData::from_foreign(data) };
I think this is clearer to read as
drop(unsafe { T::QueueData::from_foreign(data) });
> + });
> +
> // SAFETY: `bindings::queue_limits` contain only fields that are valid when zeroed.
> let mut lim: bindings::queue_limits = unsafe { core::mem::zeroed() };
>
> @@ -112,7 +120,7 @@ pub fn build<T: Operations>(
> bindings::__blk_mq_alloc_disk(
> tagset.raw_tag_set(),
> &mut lim,
> - core::ptr::null_mut(),
> + data.cast(),
> static_lock_class!().as_ptr(),
> )
> })?;
> @@ -166,8 +174,12 @@ pub fn build<T: Operations>(
> },
> )?;
>
> + recover_data.dismiss();
> +
> // INVARIANT: `gendisk` was initialized above.
> // INVARIANT: `gendisk` was added to the VFS via `device_add_disk` above.
> + // INVARIANT: `gendisk.queue.queue_data` is set to `data` in the call to
> + // `__blk_mq_alloc_disk` above.
> Ok(GenDisk {
> _tagset: tagset,
> gendisk,
> @@ -179,9 +191,10 @@ pub fn build<T: Operations>(
> ///
> /// # Invariants
> ///
> -/// - `gendisk` must always point to an initialized and valid `struct gendisk`.
> -/// - `gendisk` was added to the VFS through a call to
> -/// `bindings::device_add_disk`.
> +/// - `gendisk` must always point to an initialized and valid `struct gendisk`.
> +/// - `gendisk` was added to the VFS through a call to
> +/// `bindings::device_add_disk`.
> +/// - `self.gendisk.queue.queuedata` is initialized by a call to `ForeignOwnable::into_foreign`.
> pub struct GenDisk<T: Operations> {
> _tagset: Arc<TagSet<T>>,
> gendisk: *mut bindings::gendisk,
> @@ -193,9 +206,20 @@ unsafe impl<T: Operations + Send> Send for GenDisk<T> {}
>
> impl<T: Operations> Drop for GenDisk<T> {
> fn drop(&mut self) {
> + // SAFETY: By type invariant of `Self`, `self.gendisk` points to a valid
> + // and initialized instance of `struct gendisk`, and, `queuedata` was
> + // initialized with the result of a call to
> + // `ForeignOwnable::into_foreign`.
> + let queue_data = unsafe { (*(*self.gendisk).queue).queuedata };
> +
> // SAFETY: By type invariant, `self.gendisk` points to a valid and
> // initialized instance of `struct gendisk`, and it was previously added
> // to the VFS.
> unsafe { bindings::del_gendisk(self.gendisk) };
> +
> + // SAFETY: `queue.queuedata` was created by `GenDiskBuilder::build` with
> + // a call to `ForeignOwnable::into_foreign` to create `queuedata`.
> + // `ForeignOwnable::from_foreign` is only called here.
> + let _queue_data = unsafe { T::QueueData::from_foreign(queue_data.cast()) };
Ditto here.
drop(unsafe { T::QueueData::from_foreign(queue_data.cast()) });
Also, is this cast necessary as of
https://lore.kernel.org/all/20250711-rnull-up-v6-16-v3-13-3a262b4e2921@kernel.org/
?
> }
> }
> diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
> index 864ff379dc91..c50959d5517b 100644
> --- a/rust/kernel/block/mq/operations.rs
> +++ b/rust/kernel/block/mq/operations.rs
> @@ -6,14 +6,15 @@
>
> use crate::{
> bindings,
> - block::mq::request::RequestDataWrapper,
> - block::mq::Request,
> + block::mq::{request::RequestDataWrapper, Request},
> error::{from_result, Result},
> prelude::*,
> - types::ARef,
> + types::{ARef, ForeignOwnable},
> };
> use core::{marker::PhantomData, sync::atomic::AtomicU64, sync::atomic::Ordering};
>
> +type ForeignBorrowed<'a, T> = <T as ForeignOwnable>::Borrowed<'a>;
> +
> /// Implement this trait to interface blk-mq as block devices.
> ///
> /// To implement a block device driver, implement this trait as described in the
> @@ -26,12 +27,20 @@
> /// [module level documentation]: kernel::block::mq
> #[macros::vtable]
> pub trait Operations: Sized {
> + /// Data associated with the `struct request_queue` that is allocated for
> + /// the `GenDisk` associated with this `Operations` implementation.
> + type QueueData: ForeignOwnable;
> +
> /// Called by the kernel to queue a request with the driver. If `is_last` is
> /// `false`, the driver is allowed to defer committing the request.
> - fn queue_rq(rq: ARef<Request<Self>>, is_last: bool) -> Result;
> + fn queue_rq(
> + queue_data: ForeignBorrowed<'_, Self::QueueData>,
> + rq: ARef<Request<Self>>,
> + is_last: bool,
> + ) -> Result;
>
> /// Called by the kernel to indicate that queued requests should be submitted.
> - fn commit_rqs();
> + fn commit_rqs(queue_data: ForeignBorrowed<'_, Self::QueueData>);
>
> /// Called by the kernel to poll the device for completed requests. Only
> /// used for poll queues.
> @@ -70,7 +79,7 @@ impl<T: Operations> OperationsVTable<T> {
> /// promise to not access the request until the driver calls
> /// `bindings::blk_mq_end_request` for the request.
> unsafe extern "C" fn queue_rq_callback(
> - _hctx: *mut bindings::blk_mq_hw_ctx,
> + hctx: *mut bindings::blk_mq_hw_ctx,
> bd: *const bindings::blk_mq_queue_data,
> ) -> bindings::blk_status_t {
> // SAFETY: `bd.rq` is valid as required by the safety requirement for
> @@ -88,10 +97,20 @@ impl<T: Operations> OperationsVTable<T> {
> // reference counted by `ARef` until then.
> let rq = unsafe { Request::aref_from_raw((*bd).rq) };
>
> + // SAFETY: `hctx` is valid as required by this function.
> + let queue_data = unsafe { (*(*hctx).queue).queuedata };
> +
> + // SAFETY: `queue.queuedata` was created by `GenDisk::try_new()` with a
> + // call to `ForeignOwnable::into_pointer()` to create `queuedata`.
> + // `ForeignOwnable::from_foreign()` is only called when the tagset is
> + // dropped, which happens after we are dropped.
> + let queue_data = unsafe { T::QueueData::borrow(queue_data.cast()) };
> +
> // SAFETY: We have exclusive access and we just set the refcount above.
> unsafe { Request::start_unchecked(&rq) };
>
> let ret = T::queue_rq(
> + queue_data,
> rq,
> // SAFETY: `bd` is valid as required by the safety requirement for
> // this function.
> @@ -110,9 +129,18 @@ impl<T: Operations> OperationsVTable<T> {
> ///
> /// # Safety
> ///
> - /// This function may only be called by blk-mq C infrastructure.
> - unsafe extern "C" fn commit_rqs_callback(_hctx: *mut bindings::blk_mq_hw_ctx) {
> - T::commit_rqs()
> + /// This function may only be called by blk-mq C infrastructure. The caller
> + /// must ensure that `hctx` is valid.
> + unsafe extern "C" fn commit_rqs_callback(hctx: *mut bindings::blk_mq_hw_ctx) {
> + // SAFETY: `hctx` is valid as required by this function.
> + let queue_data = unsafe { (*(*hctx).queue).queuedata };
> +
> + // SAFETY: `queue.queuedata` was created by `GenDisk::try_new()` with a
> + // call to `ForeignOwnable::into_pointer()` to create `queuedata`.
> + // `ForeignOwnable::from_foreign()` is only called when the tagset is
> + // dropped, which happens after we are dropped.
> + let queue_data = unsafe { T::QueueData::borrow(queue_data.cast()) };
> + T::commit_rqs(queue_data)
> }
>
> /// This function is called by the C kernel. It is not currently
>
> --
> 2.47.2
>
>
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