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
