linux-kernel - Re: [PATCH v2 3/5] rust: scatterlist: Add type-state abstraction for sg

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Message-ID: <20250823143211.GB1121521@ziepe.ca>
Date: Sat, 23 Aug 2025 11:32:11 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Danilo Krummrich <dakr@...nel.org>, akpm@...ux-foundation.org,
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, lossin@...nel.org,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	abdiel.janulgue@...il.com, lyude@...hat.com, robin.murphy@....com,
	daniel.almeida@...labora.com, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] rust: scatterlist: Add type-state abstraction for
 sg_table

On Sat, Aug 23, 2025 at 10:22:47PM +0900, Alexandre Courbot wrote:

> For reasons I am not completely clear about, the number of mapped
> segments on the device side can be smaller than the number of
> scatterlists provided by the sg_table. This is highlighted by the
> documentation for `dma_map_sg_attrs` [1] ("Returns the number of mapped
> entries (which can be less than nents) on success") and `sg_dma_address`
> [2] ("You should only work with the number of sg entries dma_map_sg
> returns, or alternatively stop on the first sg_dma_len(sg) which is 0.")

> So only calling `sg_next` until we reach the end of the list carries the
> risk that we iterate on more items than we should, with the extra ones
> having their length at 0

Correct, this is misusing the API, and I don't know if the lengths are
even guarenteed to be zero. To iterate the DMA list you must use the
length of the DMA list returned by dma_map_sg() and nothing else as
the stop condition.

To repeat again, the scatterlist data structure is "optimized" and
contains two completely different lists - the CPU list and the DMA
list. The DMA list is always <= the size of the CPU list.

For all purposes they are completely seperate things and we have a
unique set of iterators and accessors for the CPU vs DMA data.

Jason