linux-kernel - Re: [PATCH v10 2/8] mm: rust: add vm_area_struct methods that require read access

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Message-ID: <CAH5fLgg9ay1tWpKo91hYXn0dmj2Gzk1mZLvf031nnTzKbQbTZA@mail.gmail.com>
Date: Thu, 5 Dec 2024 11:48:15 +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>, Jann Horn <jannh@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, 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>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v10 2/8] mm: rust: add vm_area_struct methods that require
 read access

On Thu, Dec 5, 2024 at 11:45 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Thu, Dec 05, 2024 at 11:40:03AM +0100, Alice Ryhl wrote:
> > On Fri, Nov 29, 2024 at 5:32 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
> > >
> > > This adds a type called VmAreaRef which is used when referencing a vma
> > > that you have read access to. Here, read access means that you hold
> > > either the mmap read lock or the vma read lock (or stronger).
> > >
> > > Additionally, a vma_lookup method is added to the mmap read guard, which
> > > enables you to obtain a &VmAreaRef in safe Rust code.
> > >
> > > This patch only provides a way to lock the mmap read lock, but a
> > > follow-up patch also provides a way to just lock the vma read lock.
> > >
> > > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)
> > > Reviewed-by: Jann Horn <jannh@...gle.com>
> > > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> >
> > It looks like binder needs a way to check whether a given vma is
> > associated with a given mm. I guess we can do that by adding a method
> > to VmAreaRef that returns a &MmWithUser.
>
> Presumably this would be with a lock held to ensure the VMA doesn't
> disappear from under us?
>
> I guess that's implied by possessing a VmAreaRef in the first place.
>
> I also suppoes that the mm having users is implied by you having the VMA
> that implies a lock held on it :)

That was what I was thinking.

> So that's probably fine then, as long as you can implement some sensible
> means of comparison between a known 'given mm' vs. the &MmWithUser.

We can use the stdlib function core::ptr::eq to compare the address.


Alice