Re: [net-next v1 08/16] memory-provider: dmabuf devmem memory provider

[Mina Almasry]

On Tue, Dec 12, 2023 at 7:08 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Tue, Dec 12, 2023 at 06:58:17AM -0800, Mina Almasry wrote:
>
> > Jason, we set the LSB on page_pool_iov pointers before casting it to
> > struct page pointers. The resulting pointers are not useable as page
> > pointers at all.
>
> I understand that, the second ask is about maintainability of the mm
> by using correct types.
>
> > > Perhaps you can simply avoid this by arranging for this driver to also
> > > exclusively use some special type to indicate the dual nature of the
> > > pointer and leave the other drivers as using the struct page version.
> >
> > This is certainly possible, but it requires us to rename all the page
> > pointers in the page_pool to the new type, and requires the driver
> > adding devmem TCP support to rename all the page* pointer instances to
> > the new type. It's possible but it introduces lots of code churn. Is
> > the LSB + cast not a reasonable compromise here? I feel like the trick
> > of setting the least significant bit on a pointer to indicate it's
> > something else has a fair amount of precedent in the kernel.
>
> Linus himself has complained about exactly this before, and written a cleanup:
>
> https://lore.kernel.org/linux-mm/20221108194139.57604-1-torvalds@xxxxxxxxxxxxxxxxxxxx/
>
> If you mangle a pointer *so it is no longer a pointer* then give it a
> proper opaque type so the compiler can check everything statically and
> require that the necessary converters are called in all cases.
>
> You call it churn, I call it future maintainability. :(
>
> No objection to using the LSB, just properly type a LSB mangled
> pointer so everyone knows what is going on and don't call it MM's
> struct page *.
>
> I would say this is important here because it is a large driver facing
> API surface.
>

OK, I imagine this is not that hard to implement - it's really whether
the change is acceptable to reviewers.

I figure I can start by implementing a no-op abstraction to page*:

typedef struct page netmem_t

and replace the page* in the following places with netmem_t*:

1. page_pool API (not internals)
2. drivers using the page_pool.
3. skb_frag_t.

I think that change needs to be a separate series by itself. Then the
devmem patches would on top of that change netmem_t such that it can
be a union between struct page and page_pool_iov and add the special
handling of page_pool_iov. Does this sound reasonable?


--
Thanks,
Mina