linux-kernel - Re: [PATCH 00/13] [RFC] Rust support

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Message-ID: <20210414200953.GX2531743@casper.infradead.org>
Date:   Wed, 14 Apr 2021 21:09:53 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     ojeda@...nel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/13] [RFC] Rust support

On Wed, Apr 14, 2021 at 08:45:51PM +0200, ojeda@...nel.org wrote:
>   - Manish Goregaokar implemented the fallible `Box`, `Arc`, and `Rc`
>     allocator APIs in Rust's `alloc` standard library for us.

There's a philosophical point to be discussed here which you're skating
right over!  Should rust-in-the-linux-kernel provide the same memory
allocation APIs as the rust-standard-library, or should it provide a Rusty
API to the standard-linux-memory-allocation APIs?  You seem to be doing
both ... there was a wrapper around alloc_pages() in the Binder patches,
and then you talk about Box, Arc and Rc here.

Maybe there's some details about when one can use one kind of API and
when to use another.  But I fear that we'll have Rust code at interrupt
level trying to use allocators which assume that they can sleep, and
things will go badly wrong.

By the way, I don't think that Rust necessarily has to conform to the
current way that Linux works.  If this prompted us to track the current
context (inside spinlock, handling interrupt, performing writeback, etc)
and do away with (some) GFP flags, that's not the end of the world.
We're already moving in that direction to a certain extent with the
scoped memory allocation APIs to replace GFP_NOFS / GFP_NOIO.