LinuxKPI: Add ksize() implementation

[wulf]

Summary

ksize() gets the actual amount of memory allocated for a given object and is used once in drm-kmod (drivers/dma-buf/reservation.c) to align C99 flexible array size with UMA zone item size.
This patch is borrowed from kern_malloc.c and allows to strip "#ifdef _ _FreeBSD_ _" from aforementioned drm-kmod's file. Quick test shows no positive effect from better array size alignment at least in my setup.

Test Plan

Test allocation sizes with GENERIC kernel config and with DEBUG_MEMGUARD and DEBUG_REDZONE options enabled.

[• hselasky]

Looks good to me. Let Konstantin have a say too before it goes in.

[• hselasky]

Make sure you have the word LinuxKPI in the commit message.

[kib]

I suppose this was extracted from realloc(9) ?

I dislike the addition of code that knows so much about internals of malloc(9), into linuxkpi. I suggest to move this function to kern_malloc.c as is and name it e.g. malloc_usable_size(), since this is the usermode name for the same functionality. Then linuxkpi is just a renaming macro.

sys/compat/linuxkpi/common/include/linux/slab.h

193

Can we drop all that extern from function declarations in linuxkpi ?

[wulf]

Move main routine in to kern_malloc.c
Add pair of lines to malloc(9) manpage

[wulf]

In D26215#582535, @kib wrote:

I suppose this was extracted from realloc(9) ?

Yes. it was extracted from realloc(9) and free(9)

[kib]

sys/sys/malloc.h

251

Move this line right after malloc_uninit() to keep declarations sorted by name.

sys/vm/memguard.h

49

Similarly move to after memguard_free, but this is too unordered already.