Re: [PATCH 1/2] rust: add initial scatterlist bindings

[Abdiel Janulgue <abdiel.janulgue@gmail.com>]

On 28/06/2025 14:18, Alexandre Courbot wrote:
> On Thu Jun 5, 2025 at 10:22 PM JST, Abdiel Janulgue wrote:
>>
>>
>> On 30/05/2025 17:02, 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.
>>
>>
>> Hi just a silly observation while trying to think about other ways to
>> tie the page lifetime to the sgtable. Why can't we just use a lifetime
>> bound annotation?
>>
>> It's simpler and it seems to work:
>>
>>
>> impl<'b> SGEntry<'b, Unmapped> {
>>       pub fn set_page<'a: 'b> (&mut self, page: &'a Page, length: u32,
>> offset: u32)
>>
>> So with this, my erroneous example fails to compile. Here the compiler
>> enforces the use  of the api so that the page of the lifetime is always
>> tied to the sgtable:
>>
>>
>> let sgt = sgt.init(|iter| {
>>      |                             ---- has type
>> `kernel::scatterlist::SGTableIterMut<'1>`
>> 71 |             for sg in iter {
>>      |                 -- assignment requires that borrow lasts for `'1`
>> 72 |                 sg.set_page(&Page::alloc_page(GFP_KERNEL)?,
>> PAGE_SIZE as u32, 0);
>>      |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>                - temporary value is freed at the end of this statement
>>      |                              |
>>      |                              creates a temporary value which is
>> freed while still in use
> 
> That would work for this example, but IIUC the bound lifetime will also
> prevent you from doing any sort of dynamic lifetime management using a
> smart pointer, meaning you cannot store the SGTable into another object?
> 
> Whereas storing any generic owner lets use pass a regular reference
> (which lifetime will thus propagate to the SGTable) to serve your
> example, but also works with any smart pointer.

Thanks for the explanation, indeed that's the limitation if we use a 
lifetime bound on SGTable.
Anyways, I just submitted the v2[1] which should be able to enforce the 
ownership of the pages to the SGTable.

/Abdiel

[1]https://lore.kernel.org/rust-for-linux/20250626203247.816273-1-abdiel.janulgue@gmail.com/