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. Jason
