[RFC PATCH 0/5] Rust block device driver API and null block driver
From: Andreas Hindborg
Date: Wed Mar 13 2024 - 07:07:56 EST
From: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
Hi All!
This is the second version of the Rust block device driver API and the Rust null
block driver. The context and motivation can be seen in cover letter of the RFC
v1 [1]. If more context is required, a talk about this effort was recorded at
LPC [2]. I hope to be able to discuss this series at LSF this year [3].
The series is still in RFC state, as many dependencies have not yet been merged.
Changes from v1:
- Improved request lifetime tracking
- Changed rust null block driver name to `rnull`
- Added timer completion support to `rnull`
- Added softirq completion support to `rnull`
- Moved to `xarray` for memory backing
- Adopted the folio abstraction where applicable
- Added `SAFETY` comments to all unsafe blocks
- Improved documentation and added examples
- Fixed up `vtable` default method behavior
- Dropped `unsafe` from vtable builder
- Fixed a safety issue in `RawWriter`
- Dropped the `TagSetRef` abstraction
- Renamed `Bio::iter` -> `Bio::raw_iter`
- Updated `core::fmt::Display for Bio` for readability
- Simplified `BioIterator::next`
- Documented and refactored `bvec_iter` functions
- Moved cache alignment out of `Lock`
- Added MAINTAINER entry
Thanks for all the input on v1!
Performance
===========
Rather than include an abundance of performance numbers in this letter, I would
refer to [4] for a comparison of performance to the C null block driver. In
general across all of the benchmark configurations, the mean of the difference
is -2.5%, with individual configurations showing [-10%;30%] difference in IO/s.
Request lifetime modeling
=========================
While implementing timer completion, it became clear that the request lifetime
modeling applied in the v1 RFC was not adequate. With that modeling, combined
with the timer callback feature, a developer would be able to write safe Rust
code that violates the Rust invariants for references, thus leading to undefined
behavior. A similar situation would happen in the completion path when
developers write code to convert from a completion id to a `&Request`.
To make these situations safe, and thus free from undefined behavior, the
`Request` type now applies reference counting. The logic piggybacks on the
`atomic_t ref` field of the C `struct request`. In order to make this work, the
Rust code has to be able to free requests when the refcount reaches zero.
Therefore, the series exposes (EXPORT_SYMBOL_GPL) the internal blk-mq symbol
`__blk_mq_free_request`.
I am curious what the community thinks of this approach and whether it is OK to
expose this symbol so that Rust can call it.
One consequence of these changes is that requests that are processed by a Rust
block device driver, are freed at a different place in the completion path, than
requests processed by C drivers. Requests processed by C drivers are typically
freed and recycled inline during the call to `blk_mq_complete_request`. The
requests processed by a Rust driver will be recycled when the `RequestRef` is
dropped, which is usually at some time after the request has been completed.
There does not seem to be any statistically significant effect on performance
for the rust null block implementation due to this change.
Dependencies
============
This series is based on the branch `rnull-deps-v6.8` of [5]. This tree is based
on the upstream `v6.8` tag and has been prepared with dependencies for this
series:
- [17] rust: add `CacheAligned` for easy cache line alignment of values
- [16] rust: xarray: add mutable access through `Guard`
- [15] rust: hrtimer: introduce hrtimer support
- [14] rust: add `Opaque::try_ffi_init`
- [13] rust: make `ARef` repr transparent
- [12] rust: folio: add `with_slice_into_page()`
- [11] rust: page: add `from_raw()`
- [10] rust: page: add `with_slice_into_page()`
- [ 9] IM: implement `ForeignOwnable` for `Pin`
- [ 8] IM: add `module_params` macro
- [ 7] rust: xarray: fix example
- [ 6] LIST: rust: xarray: Add an abstraction for XArray
- [ 5] LIST: rust: types: add FOREIGN_ALIGN to ForeignOwnable
- [ 4] LIST: rust: lock: implement `IrqSaveBackend` for `SpinLock`
- [ 3] LIST: rust: lock: add support for `Lock::lock_irqsave`
- [ 2] LIST: rust: add improved version of `ForeignOwnable::borrow_mut`
- [ 1] LIST: rust: folio: introduce basic support for folios
- [ 0] LIST: rust: add abstraction for `struct page`
Dependencies 0-6 are patches that have appeared on the list but are not yet
merged. Dependencies 8-9 are imports from the `rust` branch [6,7] that have not
yet been submitted. Dependencies 10-17 I will submit independently in the near
future.
If the upstream maintainers agree, then when the dependencies are merged, I will
eventually start submitting PRs to Jens. I fully commit to maintain this code as
indicated in the MAINTAINERS entry.
Best regards,
Andreas Hindborg
[1] https://lore.kernel.org/rust-for-linux/20230503090708.2524310-1-nmi@xxxxxxxxxxxx
[2] https://lpc.events/event/17/contributions/1425
[3] https://lore.kernel.org/rust-for-linux/87v87cgsp8.fsf@xxxxxxxxxxxx
[4] https://rust-for-linux.com/null-block-driver
[5] git https://github.com/metaspace/linux.git rnull-deps-v6.8
[6] git https://github.com/rust-for-Linux/linux.git rust
[7] https://rust-for-linux.com/branches#rust
Andreas Hindborg (5):
rust: block: introduce `kernel::block::mq` module
rust: block: introduce `kernel::block::bio` module
rust: block: allow `hrtimer::Timer` in `RequestData`
rust: block: add rnull, Rust null_blk implementation
MAINTAINERS: add entry for Rust block device driver API
MAINTAINERS | 14 ++
block/blk-mq.c | 3 +-
drivers/block/Kconfig | 4 +
drivers/block/Makefile | 3 +
drivers/block/rnull.rs | 323 +++++++++++++++++++++++++++
include/linux/blk-mq.h | 1 +
rust/bindings/bindings_helper.h | 2 +
rust/helpers.c | 46 ++++
rust/kernel/block.rs | 6 +
rust/kernel/block/bio.rs | 112 ++++++++++
rust/kernel/block/bio/vec.rs | 279 +++++++++++++++++++++++
rust/kernel/block/mq.rs | 131 +++++++++++
rust/kernel/block/mq/gen_disk.rs | 174 +++++++++++++++
rust/kernel/block/mq/operations.rs | 346 +++++++++++++++++++++++++++++
rust/kernel/block/mq/raw_writer.rs | 60 +++++
rust/kernel/block/mq/request.rs | 269 ++++++++++++++++++++++
rust/kernel/block/mq/tag_set.rs | 117 ++++++++++
rust/kernel/error.rs | 5 +
rust/kernel/lib.rs | 1 +
scripts/Makefile.build | 2 +-
20 files changed, 1896 insertions(+), 2 deletions(-)
create mode 100644 drivers/block/rnull.rs
create mode 100644 rust/kernel/block.rs
create mode 100644 rust/kernel/block/bio.rs
create mode 100644 rust/kernel/block/bio/vec.rs
create mode 100644 rust/kernel/block/mq.rs
create mode 100644 rust/kernel/block/mq/gen_disk.rs
create mode 100644 rust/kernel/block/mq/operations.rs
create mode 100644 rust/kernel/block/mq/raw_writer.rs
create mode 100644 rust/kernel/block/mq/request.rs
create mode 100644 rust/kernel/block/mq/tag_set.rs
base-commit: e8f897f4afef0031fe618a8e94127a0934896aba
prerequisite-patch-id: 299c2cc48c45c8149e7cb18c6146a0308e5d0a44
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prerequisite-patch-id: aef3042976c4c678b7aa96154fc280f9061ebaf7
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prerequisite-patch-id: 167c7770e124b9afa44bead742f90a57ac73f2d7
prerequisite-patch-id: cc24a3135b4f258f4b1ea83ac91c2be4ffe31772
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2.44.0