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Date: Sat, 16 Mar 2024 22:48:26 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Andreas Hindborg <nmi@metaspace.dk>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Jens Axboe <axboe@kernel.dk>, Keith Busch <kbusch@kernel.org>,
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Subject: Re: [RFC PATCH 1/5] rust: block: introduce `kernel::block::mq` module
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On Fri, Mar 15, 2024 at 06:49:39PM +0100, Andreas Hindborg wrote:
> Ming Lei <ming.lei@redhat.com> writes:
> 
> > On Fri, Mar 15, 2024 at 01:46:30PM +0100, Andreas Hindborg wrote:
> >> Ming Lei <ming.lei@redhat.com> writes:
> >> > On Fri, Mar 15, 2024 at 08:52:46AM +0100, Andreas Hindborg wrote:
> >> >> Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> writes:
> >> >> 
> >> >> > On Thu, Mar 14, 2024 at 8:23 PM Andreas Hindborg <nmi@metaspace.dk> wrote:
> >> >> >>
> >> >> >> The way the current code compiles, <kernel::block::mq::Request as
> >> >> >> kernel::types::AlwaysRefCounted>::dec_ref` is inlined into the `rnull`
> >> >> >> module. A relocation for `rust_helper_blk_mq_free_request_internal`
> >> >> >> appears in `rnull_mod.ko`. I didn't test it yet, but if
> >> >> >> `__blk_mq_free_request` (or the helper) is not exported, I don't think
> >> >> >> this would be possible?
> >> >> >
> >> >> > Yeah, something needs to be exported since there is a generic
> >> >> > involved, but even if you want to go the route of exporting only a
> >> >> > different symbol, you would still want to put it in the C header so
> >> >> > that you don't get the C missing declaration warning and so that we
> >> >> > don't have to write the declaration manually in the helper.
> >> >> 
> >> >> That is what I did:
> >> >> 
> >> >> @@ -703,6 +703,7 @@ int blk_mq_alloc_sq_tag_set(struct blk_mq_tag_set *set,
> >> >>  		unsigned int set_flags);
> >> >>  void blk_mq_free_tag_set(struct blk_mq_tag_set *set);
> >> >>  
> >> >> +void __blk_mq_free_request(struct request *rq);
> >> >>  void blk_mq_free_request(struct request *rq);
> >> >>  int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,
> >> >>  		unsigned int poll_flags);
> >> >
> >> > Can you explain in detail why one block layer internal helper is
> >> > called into rnull driver directly? It never happens in C driver code.
> >> 
> >> It is not the rust null block driver that calls this symbol directly. It
> >> is called by the Rust block device driver API. But because of inlining,
> >> the symbol is referenced from the loadable object.
> >
> > What is the exact Rust block device driver API? The key point is that how
> > the body of one exported kernel C API(EXPORT_SYMBOL) becomes inlined
> > with Rust driver.
> 
> This happens when `ARef<Request<_>>` is dropped. The drop method
> (destructor) of this smart pointer decrements the refcount and
> potentially calls `__blk_mq_free_request`.
> 
> >> 
> >> The reason we have to call this symbol directly is to ensure proper
> >> lifetime of the `struct request`. For example in C, when a driver
> >
> > Sounds Rust API still calls into __blk_mq_free_request() directly, right?
> 
> Yes, the Rust block device driver API will call this request if an
> `ARef<Request<_>>` is dropped and the refcount goes to 0.
> 
> > If that is the case, the usecase need to be justified, and you need
> > to write one standalone patch with the exact story for exporting
> > __blk_mq_free_request().
> 
> Ok, I can do that.
> 
> >
> >> converts a tag to a request, the developer makes sure to only ask for
> >> requests which are outstanding in the driver. In Rust, for the API to be
> >> sound, we must ensure that the developer cannot write safe code that
> >> obtains a reference to a request that is not owned by the driver.
> >> 
> >> A similar issue exists in the null block driver when timer completions
> >> are enabled. If the request is cancelled and the timer fires after the
> >> request has been recycled, there is a problem because the timer holds a
> >> reference to the request private data area.
> >> 
> >> To that end, I use the `atomic_t ref` field of the C `struct request`
> >> and implement the `AlwaysRefCounted` Rust trait for the request type.
> >> This is a smart pointer that owns a reference to the pointee. In this
> >> way, the request is not freed and recycled until the smart pointer is
> >> dropped. But if the smart pointer holds the last reference when it is
> >> dropped, it must be able to free the request, and hence it has to call
> >> `__blk_mq_free_request`.
> >
> > For callbacks(queue_rq, timeout, complete) implemented by driver, block
> > layer core guaranteed that the passed request reference is live.
> >
> > So driver needn't to worry about request lifetime, same with Rust
> > driver, I think smart pointer isn't necessary for using request in
> > Rust driver.
> 
> Using the C API, there is nothing preventing a driver from using the
> request after the lifetime ends.

Yes, it is true for C, so will Rust-for-linux need to add refcount for
most exported kernel C structure? such as by implementing AlwaysRefCounted
traits?

> With Rust, we have to make it
> impossible.Without the refcount and associated call to
> `__blk_mq_free_request`, it would be possible to write Rust code that
> access the request after the lifetime ends. This is not sound, and it is
> something we need to avoid in the Rust abstractions.
> 
> One concrete way to do write unsound code with a Rust API where lifetime
> is not tracked with refcount, is if the null block timer completion
> callback fires after the request is completed. Perhaps the driver
> cancels the request but forgets to cancel the timer. When the timer
> fires, it will access the request via the context pointer, but the
> request will be invalid.

The issue is less serious for blk-mq request, which is pre-allocated,
and one freed request just means it can be re-allocated for other IO
in same queue, and the pointed memory won't be really freed.

Also as I mentioned, inside driver's ->timeout(), the request is
guaranteed to be live by block layer core(won't be re-allocated to other IO),
the passed-in request is referenced already, please see bt_iter() which
is called from blk_mq_timeout_work(). Here, block layer core just
borrows request, then passed the reference to ->timeout(), when
request is owned by driver actually.

I understand Rust block driver still need to implement ->queue_rq(),
->timeout(), ..., just like C driver, but maybe I am wrong? Or Rust block driver
will re-implement part of block layer core code? such as, get one extra
reference of request no matter block core has done that.

> In C we have to write the driver code so this
> cannot happen. In Rust, the API must prevent this from happening. So any
> driver written in the safe subset of Rust using this API can never
> trigger this behavior.
> 
> By using the refcount, we ensure that the request is alive until all
> users who hold a reference to it are dropped.

block layer has provided such guarantee if Rust driver follows current
block driver model.

> 
> Another concrete example is when a driver calls `blk_mq_tag_to_rq` with
> an invalid tag. This can return a reference to an invalid tag, if the
> driver is not implemented correctly. By using `req_ref_inc_not_zero` we
> can assert that the request is live before we create a Rust reference to
> it, and even if the driver code has bugs, it can never access an invalid
> request, and thus it can be memory safe.
> 
> We move the responsibility of correctness, in relation to memory safety,
> from the driver implementation to the API implementation.

After queue_rq(req) is called, request ownership is actually transferred to
driver like Rust's move, then driver is free to call blk_mq_tag_to_rq(), and
finally return request to block core after the request is completed by driver.

The biggest question should be how Rust block driver will be designed &
implemented? Will rust block driver follow current C driver's model, such
as implementing ->queue_rq(), ->timeout(), ->complete()...?


thanks,
Ming