On Thu, Nov 21, 2024 at 11:10:45AM -0800, Boqun Feng wrote:
> [Cc Kairui in case he's interested]
>
(forgot to cc...)
> On Thu, Nov 21, 2024 at 11:30:13AM +0200, Abdiel Janulgue wrote:
> > Hi Boqun, Matthew:
> >
> > On 21/11/2024 02:24, Boqun Feng wrote:
> > > > > So if I understand correctly, what Abdiel needs here is a way to convert
> > > > > a virtual address to the corresponding page, would it make sense to just
> > > > > use folio in this case? Abdiel, what's the operation you are going to
> > > > > call on the page you get?
> > > >
> > > > Yes that's basically it. The goal here is represent those existing struct
> > > > page within this rust Page abstraction but at the same time to avoid taking
> > > > over its ownership.
> > > >
> > > > Boqun, Alice, should we reconsider Ownable and Owned trait again? :)
> > > >
> > >
> > > Could you use folio in your case? If so, we can provide a simple binding
> > > for folio which should be `AlwaysRefcounted`, and re-investigate how
> > > page should be wrapped.
> > >
> >
> > I'm not sure. Is there a way to get the struct folio from a vmalloc'd
> > address, e.g vmalloc_to_folio()?
> >
>
> I think you can use page_folio(vmalloc_to_page(..)) to get the folio,
> but one thing to notice is that folio is guaranteed to be a non-tail
> page, so if you want to do something later for the particular page (if
> it's a tail page), you will need to know the offset of the that page in
> folio. You can do something like below:
>
> pub fn page_slice_to_folio<'a>(page: &PageSlice) -> Result<(&'a Folio, usize)> {
> ...
> let page = vmalloc_to_page(ptr);
>
> let folio = page_folio(page);
> let offset = folio_page_idx(folio, page);
>
> Ok((folio, offset))
> }
>
> And you have a folio -> page function like:
>
> pub struct Folio(Opaque<bindings::folio>);
>
> impl Folio {
> pub fn nth_page(&self, n: usize) -> &Page {
> &*(nth_page(self.0.get(), n))
> }
> }
>
> Of course, this is me acting as I know MM ;-) but I feel this is the way
> to go. And if binder can use folio as well (I don't see a reason why
> not, but it's extra work, so defer to Alice), then we would only need
> the `pub struct Page { inner: Opaque<bindings::page> }` part in your
> patch #1, and can avoid doing `Ownable` or `AlwaysRefcounted` for
> `Page`.
>
> Thoughts?
>
> Regards,
> Boqun
>
> > Regards,
> > Abdiel
