linux-kernel - Re: [PATCH v3] io: add io

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Message-ID: <cfce9ad9-c518-44c7-a602-8ce7c3fffb2b@arm.com>
Date: Mon, 1 Dec 2025 13:55:34 +0000
From: Robin Murphy <robin.murphy@....com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Will Deacon <will@...nel.org>,
 Daniel Almeida <daniel.almeida@...labora.com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 Joerg Roedel <joro@...tes.org>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Asahi Lina <lina+kernel@...hilina.net>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, iommu@...ts.linux.dev, linux-mm@...ck.org
Subject: Re: [PATCH v3] io: add io_pgtable abstraction

On 2025-12-01 9:58 am, Alice Ryhl wrote:
[...]
>>> We need a different signature if it's possible to have mapped != 0 when
>>> returning an error.
>>
>> Aha, thanks for clarifying - indeed this is not the common "value or error"
>> case, it is two (almost) orthogonal return values. However if we're not
>> permitting callers to try to do anything clever with -EEXIST then it might
>> make sense to just embed the inevitable cleanup-on-failure boilerplate here
>> anyway (even if we still leave retry-on-partial-success to the caller).
> 
> Is the only possible error -EEXIST? I could encode that in the API if
> that is the case.

No, I was just calling out -EEXIST as the only error where I imagine a 
caller *might* want to continue without cleaning up a partial mapping, 
if for instance they were playing clever tricks like a background 
mapping of a large buffer while already allowing other threads to 
eagerly demand-page bits of it, so the "main" mapping thread just 
adjusts and restarts to skip over already-present pages. Other errors 
are still possible, but generally represent terminal failure conditions 
at the caller's level too - in practice things like -EINVAL and -ENOMEM 
are likely to happen before any mappings can be made, but io-pgtable 
doesn't guarantee any particular behaviour here, so a well-behaved 
caller should still generally handle cleaning up after an error (at 
least if they intend to keep trying to use the pagetable beyond that point).

Cheers,
Robin.