EXPORT_SYMBOL causes undefined reference in one direction, but not the other (relocation truncated)

I'm working on the 4.9 Linux kernel on a aarch64 machine, specifically mm/memory.c and a custom platform device driver. My goal is to have my device driver communicate some information to hardware that originates in functions in memory.c.

At first, I tried the same approach I always use for communicating across (platform) device drivers:

• EXPORT_SYMBOL for the respective function in driver A
• Defining the symbol as extern in driver B and accessing

Usually works like a charm, but this time I came across the following error when linking, with the platform driver EXPORTing and memory.c using extern:

mm/memory.c:164:(.text+0x2a874): relocation truncated to fit: R_AARCH64_ADR_PREL_PG_HI21 against undefined symbol `my_func'

However, if I do the opposite, that is:

• Define and EXPORT_SYMBOL a function pointer in memory.c
• Assign a pointer to the platform driver function to the exported symbol

... it works!

Specifically...

Platform Driver:

void my_func(args){ ... };
EXPORT_SYMBOL(my_func);

memory.c:

extern void my_func(args);

... causes the linker error described above.

But ...

Platform Driver:

extern void (*funcptr)(args);

driver_probe() {
...
funcptr = &my_func;
....
}

memory.c:

void (*funcptr)(args) = NULL;
EXPORT_SYMBOL(funcptr);

... works !

A quick google search hinted that the linker error is concerned with the gcc options PIC/PIE, but I couldn't find a definitive answer.

It works now ... but WHY? :-)