Re: [PATCH v8 4/7] mm: rust: add lock_vma_under_rcu

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Nov 20, 2024 at 8:29 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Wed, Nov 20, 2024 at 02:49:58PM +0000, Alice Ryhl wrote:
> > All of Rust Binder's existing calls to `vm_insert_page` could be
> > optimized to first attempt to use `lock_vma_under_rcu`. This patch
> > provides an abstraction to enable that.
>
> I think there should be a blurb about what the VMA locks are, how they avoid
> contention on the mmap read lock etc. before talking about a use case (though
> it's useful to mention the motivating reason!)
>
> >
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/helpers/mm.c |  5 +++++
> >  rust/kernel/mm.rs | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 61 insertions(+)
> >
> > diff --git a/rust/helpers/mm.c b/rust/helpers/mm.c
> > index 7b72eb065a3e..81b510c96fd2 100644
> > --- a/rust/helpers/mm.c
> > +++ b/rust/helpers/mm.c
> > @@ -43,3 +43,8 @@ struct vm_area_struct *rust_helper_vma_lookup(struct mm_struct *mm,
> >  {
> >       return vma_lookup(mm, addr);
> >  }
> > +
> > +void rust_helper_vma_end_read(struct vm_area_struct *vma)
> > +{
> > +     vma_end_read(vma);
> > +}
> > diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
> > index ace8e7d57afe..a15acb546f68 100644
> > --- a/rust/kernel/mm.rs
> > +++ b/rust/kernel/mm.rs
> > @@ -13,6 +13,7 @@
> >  use core::{ops::Deref, ptr::NonNull};
> >
> >  pub mod virt;
> > +use virt::VmAreaRef;
> >
> >  /// A wrapper for the kernel's `struct mm_struct`.
> >  ///
> > @@ -170,6 +171,32 @@ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser {
> >          unsafe { &*ptr.cast() }
> >      }
> >
> > +    /// Try to lock the vma read lock under rcu.
>
> This reads oddly, I'd say 'try to acquire the VMA read lock'. It's not really
> necessary to mention RCU here I'd say, as while lock_vma_under_rcu() acquires
> the RCU lock in order to try to get the VMA read lock, it releases it afterwards
> and you hold the VMA read luck until you are done with it and don't need to hold
> an RCU lock.
>
> A reader might otherwise be confused and think an RCU read lock is required to
> be held throughout too which isn't the case (this is maybe a critique of the
> name of the function too, sorry Suren :P).
>
> > +    /// If this operation fails, the vma may still exist. In that case, you should take the mmap
> > +    /// read lock and try to use `vma_lookup` instead.
>
> This also reads oddly, you're more likely (assuming you are not arbitrarily
> trying to acquire a lock on an address likely to be unmapped soon) to have
> failed due to lock contention.
>
> So I'd say 'this is an optimistic try lock operation, so it may fail, in which
> case you should fall back to taking the mmap read lock'.
>
> I'm not sure it's necessary to reference vma_lookup() either, especially as in
> future versions of this code we might want to use a VMA iterator instead.

Thanks for the doc suggestions, they sound great.

> > +    ///
> > +    /// When per-vma locks are disabled, this always returns `None`.
> > +    #[inline]
> > +    pub fn lock_vma_under_rcu(&self, vma_addr: usize) -> Option<VmaReadGuard<'_>> {
>
> Ah I love having lock guards available... Something I miss from C++ :>)

I've heard that C is starting to get lock guards recently!

> > +        #[cfg(CONFIG_PER_VMA_LOCK)]
>
> Ah interesting, so we have an abstraction for kernel config operations!

Yeah, it's basically an #ifdef, but the block must still parse even if
the config is disabled.

Alice