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

[Alice Ryhl <aliceryhl@google.com>]

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?

Alice