Re: [PATCH v3 0/2] rust: page: Add support for existing struct page mappings

[Boqun Feng <boqun.feng@gmail.com>]

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