Re: [PATCH v9 2/8] mm: rust: add vm_area_struct methods that require read access

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Nov 27, 2024 at 4:41 PM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Nov 27, 2024 at 1:01 PM Alice Ryhl <aliceryhl@google.com> wrote:
> > On Tue, Nov 26, 2024 at 11:10 PM Jann Horn <jannh@google.com> wrote:
> > >
> > > On Fri, Nov 22, 2024 at 4:41 PM Alice Ryhl <aliceryhl@google.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@oracle.com> (for mm bits)
> > > > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > >
> > > Reviewed-by: Jann Horn <jannh@google.com>
> >
> > Thanks!
> >
> > > with one comment:
> > >
> > > > +    /// Zap pages in the given page range.
> > > > +    ///
> > > > +    /// This clears page table mappings for the range at the leaf level, leaving all other page
> > > > +    /// tables intact, and freeing any memory referenced by the VMA in this range. That is,
> > > > +    /// anonymous memory is completely freed, file-backed memory has its reference count on page
> > > > +    /// cache folio's dropped, any dirty data will still be written back to disk as usual.
> > > > +    #[inline]
> > > > +    pub fn zap_page_range_single(&self, address: usize, size: usize) {
> > > > +        // SAFETY: By the type invariants, the caller has read access to this VMA, which is
> > > > +        // sufficient for this method call. This method has no requirements on the vma flags. Any
> > > > +        // value of `address` and `size` is allowed.
> > >
> > > If we really want to allow any address and size, we might want to add
> > > an early bailout in zap_page_range_single(). The comment on top of
> > > zap_page_range_single() currently says "The range must fit into one
> > > VMA", and it looks like by the point we reach a bailout, we could have
> > > gone through an interval tree walk via
> > > mmu_notifier_invalidate_range_start()->__mmu_notifier_invalidate_range_start()->mn_itree_invalidate()
> > > for a range that ends before it starts; I don't know how safe that is.
> >
> > I could change the comment on zap_page_range_single() to say:
> >
> > "The range should be contained within a single VMA. Otherwise an error
> > is returned."
> >
> > And then I can add an overflow check at the top of
> > zap_page_range_single(). Sounds ok?
>
> Yes, I think changing the comment like that and adding a check for
> whether address+size wraps around there addresses this.

Can there be a page at the top of the address space? If so, I have to
be a bit careful in the wrap-around check, because it should only fail
if the addition wraps around *and* the sum is non-zero.

Alice