Message-ID: <95ff963ddabf7c3cd2cfd07d0231a0073ff6847e.camel@redhat.com>
Date: Wed, 04 Jun 2025 14:21:48 -0400
From: Lyude Paul <lyude@...hat.com>
To: Alexandre Courbot <acourbot@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>,
  Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: dakr@...nel.org, Miguel Ojeda <ojeda@...nel.org>, 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>, Alice Ryhl <aliceryhl@...gle.com>, Trevor
 Gross <tmgross@...ch.edu>,  Valentin Obst <kernel@...entinobst.de>, open
 list <linux-kernel@...r.kernel.org>, Marek Szyprowski	
 <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, 
	airlied@...hat.com, rust-for-linux@...r.kernel.org, "open list:DMA MAPPING
 HELPERS" <iommu@...ts.linux.dev>, Petr Tesarik <petr@...arici.cz>, Andrew
 Morton	 <akpm@...ux-foundation.org>, Herbert Xu
 <herbert@...dor.apana.org.au>, Sui Jingfeng <sui.jingfeng@...ux.dev>, Randy
 Dunlap <rdunlap@...radead.org>, Michael Kelley	 <mhklinux@...look.com>
Subject: Re: [PATCH 1/2] rust: add initial scatterlist bindings

On Fri, 2025-05-30 at 23:02 +0900, Alexandre Courbot wrote:
> On Thu May 29, 2025 at 9:45 AM JST, Jason Gunthorpe wrote:
> > On Thu, May 29, 2025 at 01:14:05AM +0300, Abdiel Janulgue wrote:
> > > +impl SGEntry<Unmapped> {
> > > +    /// Set this entry to point at a given page.
> > > +    pub fn set_page(&mut self, page: &Page, length: u32, offset: u32) {
> > > +        let c: *mut bindings::scatterlist = self.0.get();
> > > +        // SAFETY: according to the `SGEntry` invariant, the scatterlist pointer is valid.
> > > +        // `Page` invariant also ensures the pointer is valid.
> > > +        unsafe { bindings::sg_set_page(c, page.as_ptr(), length, offset) };
> > > +    }
> > > +}
> > 
> > Wrong safety statement. sg_set_page captures the page.as_ptr() inside
> > the C datastructure so the caller must ensure it holds a reference on
> > the page while it is contained within the scatterlist.
> > 
> > Which this API doesn't force to happen.
> > 
> > Most likely for this to work for rust you have to take a page
> > reference here and ensure the page reference is put back during sg
> > destruction. A typical normal pattern would 'move' the reference from
> > the caller into the scatterlist.
> 
> As Jason mentioned, we need to make sure that the backing pages don't get
> dropped while the `SGTable` is alive. The example provided unfortunately fails
> to do that:
> 
>     let sgt = SGTable::alloc_table(4, GFP_KERNEL)?;
>     let sgt = sgt.init(|iter| {
>         for sg in iter {
>             sg.set_page(&Page::alloc_page(GFP_KERNEL)?, PAGE_SIZE as u32, 0);
>         }
>         Ok(())
>     })?;
> 
> Here the allocated `Page`s are dropped immediately after their address is
> written by `set_page`, giving the device access to memory that may now be used
> for completely different purposes. As long as the `SGTable` exists, the memory
> it points to must not be released or reallocated in any way.
> 
> To that effect, we could simply store the `Page`s into the `SGTable`, but that
> would cover only one of the many ways they can be constructed. For instance we
> may want to share a `VVec` with a device and this just won't allow doing it.
> 
> So we need a way to keep the provider of the pages alive into the `SGTable`,
> while also having a convenient way to get its list of pages. Here is rough idea
> for doing this, it is very crude and probably not bulletproof but hopefully it
> can constitute a start.
> 
> You would have a trait for providing the pages and their range:
> 
>     /// Provides a list of pages that can be used to build a `SGTable`.
>     trait SGTablePages {
>         /// Returns an iterator to the pages providing the backing memory of `self`.
>         fn pages_iter<'a>(&'a self) -> impl Iterator<Item = &'a bindings::page>;
>         /// Returns the effective range of the mapping.
>         fn range(&self) -> Range<usize>;
>     }
> 
> The `SGTable` becomes something like:
> 
>     struct SGTable<P: SGTablePages, T: MapState>
>     {
>         table: Opaque<bindings::sg_table>,
>         pages: P,
>         _s: PhantomData<T>,
>     }

Hopefully I'm not missing anything here but - I'm not sure how I feel about
this making assumptions about the memory layout of an sg_table beyond just
being a struct sg_table. For instance, in the gem shmem helpers I had this for
exposing the SGTable that is setup for gem shmem objects:

struct OwnedSGTable<T: drm::gem::shmem::DriverObject> {
    sg_table: NonNull<SGTable>
    _owner: ARef<Object<T>>
}

So, I'm not really sure we have any reasonable representation for P here as we
don't handle the memory allocation for the SGTable.

> 
> You can then implement `SGTablePages` on anything you want to DMA map. Say a
> list of pages (using newtype on purpose):
> 
>     struct PagesArray(KVec<Page>);
> 
>     impl SGTablePages for PagesArray {
>         fn pages_iter<'a>(&'a self) -> impl Iterator<Item = &'a bindings::page> {
>             self.0.iter().map(|page| unsafe { &*page.as_ptr() })
>         }
> 
>         fn range(&self) -> Range<usize> {
>             0..(PAGE_SIZE * self.0.len())
>         }
>     }
> 
> Or a pinned `VVec`:
> 
>     impl<T> SGTablePages for Pin<VVec<T>> {
>         fn pages_iter<'a>(&'a self) -> impl Iterator<Item = &'a bindings::page> {
>             // Number of pages covering `self`
>             (0..self.len().next_multiple_of(PAGE_SIZE))
>                 .into_iter()
>                 // pointer to virtual address of page
>                 .map(|i| unsafe { self.as_ptr().add(PAGE_SIZE * i) } as *const c_void)
>                 // convert virtual address to page
>                 .map(|ptr| unsafe { &*bindings::vmalloc_to_page(ptr) })
>         }
> 
>         fn range(&self) -> Range<usize> {
>             0..self.len()
>         }
>     }
> 
> You can store these into `SGTable::pages` and ensure (unless I missed
> something) that its memory stays valid, while providing the material to
> initialize the `sg_table`.
> 
> `SGTable` could provide an accessor to `pages` so the CPU can read/write the
> data when DMA is not active (maybe also calling `dma_sync_*` as appropriate?).
> Or maybe users could put the backing object behind a smart pointer for
> concurrent accesses and pass that to `SGTable`.
> 
> One nice thing with this approach is that users don't have to figure out
> themselves how to obtain the page list for their buffer if it already has a
> `SGTablePages` implementation, like `VVec` does.
> 
> Note that although the code above builds for me, I probably got a few things
> wrong - maybe `SGTablePages` should be `unsafe`, maybe also I am misusing
> `Pin`, or overlooked a few usecases that would be impossible to implement using
> this scheme. Hopefully we can get more feedback to validate or reject this
> idea.
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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