On Thu, 21 Dec 2023 15:44:22 -0800 Mina Almasry wrote: > The warning is like so: > > ./include/net/page_pool/helpers.h: In function ‘page_pool_alloc’: > ./include/linux/stddef.h:8:14: warning: returning ‘void *’ from a > function with return type ‘netmem_ref’ {aka ‘long unsigned int’} makes > integer from pointer without a cast [-Wint-conversion] > 8 | #define NULL ((void *)0) > | ^ > ./include/net/page_pool/helpers.h:132:24: note: in expansion of macro > ‘NULL’ > 132 | return NULL; > | ^~~~ > > And happens in all the code where: > > netmem_ref func() > { > return NULL; > } > > It's fixable by changing the return to `return (netmem_ref NULL);` or > `return 0;`, but I feel like netmem_ref should be some type which > allows a cast from NULL implicitly. Why do you think we should be able to cast NULL implicitly? netmem_ref is a handle, it could possibly be some form of an ID in the future, rather than a pointer. Or have more low bits stolen for specific use cases. unsigned long, and returning 0 as "no handle" makes perfect sense to me. Note that 0 is a special case, bitwise types are allowed to convert to 0/bool and 0 is implicitly allowed to become a bitwise type. This will pass without a warning: typedef unsigned long __bitwise netmem_ref; netmem_ref some_code(netmem_ref ref) { // direct test is fine if (!ref) // 0 "upgrades" without casts return 0; // 1 does not, we need __force return (__force netmem_ref)1 | ref; } The __bitwise annotation will make catching people trying to cast to struct page * trivial. You seem to be trying hard to make struct netmem a thing. Perhaps you have a reason I'm not getting?