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

From: Jason Gunthorpe <[email-masked]>
Date: Tue, 12 Dec 2023 11:08:34 -0400

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/[email-masked]/

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.

Jason