linux-kernel - Re: [PATCH v8 4/7] mm: rust: add lock_vma_under

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Message-ID: <fe7056d9-ad54-48a5-912c-ba19ba7c09d3@lucifer.local>
Date: Thu, 21 Nov 2024 12:59:21 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.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 4/7] mm: rust: add lock_vma_under_rcu

On Thu, Nov 21, 2024 at 11:44:52AM +0100, Alice Ryhl wrote:
> On Wed, Nov 20, 2024 at 8:29 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.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!)

^ I think we should update the commit message to add this at the start then the
binder stuff underneath.

> >
> > >
> > > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

Other than the doc stuff, this looks fine, so:

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)

> > > ---
> > >  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.

Thanks :)

>
> > > +    ///
> > > +    /// 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!

Yeah there are some (e.g. [0]) but the weak typing hinders things imo and the
syntax is not fun.

This is wrt to the _kernel_ C rather than C in general though in case you were
referring to the newer standard or such!

[0]: https://elixir.bootlin.com/linux/v6.12/source/include/linux/cleanup.h#L307

>
> > > +        #[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.
>

Right, kinda sane to actually make sure it parses too... :)

> Alice