Re: [PATCH v8 5/7] mm: rust: add mmput_async support

[Lorenzo Stoakes]

On Thu, Nov 21, 2024 at 12:39:37PM +0100, Alice Ryhl wrote:
> On Wed, Nov 20, 2024 at 8:47 PM Lorenzo Stoakes
> <lorenzo.stoakes@xxxxxxxxxx> 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.

Yeah, seems not so much :>)

>
> > >  // 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_.

Ack.

>
> Alice