linux-kernel - Re: [PATCH v8 5/7] mm: rust: add mmput

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Message-ID: <CAH5fLgjDfSL+7XB28sYmq4e40yxBv97h7jQPs=9Qe4+_3f1FRw@mail.gmail.com>
Date: Thu, 21 Nov 2024 12:39:37 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Vlastimil Babka <vbabka@...e.cz>, John Hubbard <jhubbard@...dia.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Christian Brauner <brauner@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	rust-for-linux@...r.kernel.org, Andreas Hindborg <a.hindborg@...nel.org>
Subject: Re: [PATCH v8 5/7] mm: rust: add mmput_async support

On Wed, Nov 20, 2024 at 8:47 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Wed, Nov 20, 2024 at 02:49:59PM +0000, Alice Ryhl wrote:
> > Adds an MmWithUserAsync type that uses mmput_async when dropped but is
> > otherwise identical to MmWithUser. This has to be done using a separate
> > type because the thing we are changing is the destructor.
> >
> > Rust Binder needs this to avoid a certain deadlock. See commit
> > 9a9ab0d96362 ("binder: fix race between mmput() and do_exit()") for
> > details. It's also needed in the shrinker to avoid cleaning up the mm in
> > the shrinker's context.
>
> Oh Lord, I didn't even know this existed... I see it was (re-)added in commit
> a1b2289cef92 ("android: binder: drop lru lock in isolate callback") back in 2017
> so quite a history of being necessary for binder.
>
> I see mmdrop_async(), I guess that's not needed for anything binder-ish? A quick
> look in the code suggests this is invoked in free_signal_struct() and is there
> due to some softirq stuff on x86... so yeah I guess not :)

I didn't know it was so binder-specific. I assumed it would be a
relatively common use-case.

> >  // These methods are safe to call even if `mm_users` is zero.
> >  impl Mm {
> >      /// Call `mmgrab` on `current.mm`.
> > @@ -171,6 +213,13 @@ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser {
> >          unsafe { &*ptr.cast() }
> >      }
> >
> > +    /// Use `mmput_async` when dropping this refcount.
> > +    #[inline]
> > +    pub fn use_mmput_async(me: ARef<MmWithUser>) -> ARef<MmWithUserAsync> {
>
> Again, nitty, but I wonder if this should be as_xxx()?
>
> But I guess this makes sense too.

In this case, the as_ prefix is incorrect because this is an owned ->
owned conversion. See the API guidelines:
https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv

If we wish to use a prefix, the correct prefix is into_.

Alice