quoted

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I looked at that patch.

I don't think you can just drop the #ifdef in function
__access_remote_vm() in mm/memory.c

You have to replace it with something like:

     if (!IS_ENABLED(CONFIG_HAVE_IOREMAP_PROT))
         break;

Another thing in that patch:

By making generic_access_phys() a static inline, it means that everytime
you refer to the address of that function in a vm_operations_struct
struct, the compiler has to provide an outlined instance of the
function. It means you'll likely have several instances of a
generic_access_phys().

What you could do instead is to add the following at the start of
generic_access_phys() in mm/memory.c :

       if (!IS_ENABLED(CONFIG_HAVE_IOREMAP_PROT))
               return 0;

It is really a better chmoce, thanks for the advice.
Multiple instances exist as you mentioned, the block returns 0 with no-op
instance which makes no difference with the function return value.

I will update the patch after a re-confirming.

I tried as adviced but when not defined, error happens on archectures such
as arm64. Actually the function generic_access_phys calls a lot of functions
that become undefined if we compile it with CONFIG_HAVE_IOREMAP_PROT disabled.
The archectures that support CONFIG_HAVE_IOREMAP_PROT are mips, x86, sh, arc,
s390, loongarch and powerpc.

So we may just define the function with static inline and add IS_ENABLED
condition branch in function __access_remote_vm in mm/memory.c. The executing
path breaks if CONFIG_HAVE_IOREMAP_PROT is disabled, and never goes into the
static no-op function.

In short, the static inline no-op function would never be executed, the only
difference is that there would be a lot of function code in compiled target.

Thanks,
Wenhu