MAX_PHYSMEM_BITS is a definition for sparsemem, and is only visible
when sparsemem is enabled.  When sparsemem is disabled, asm/sparsemem.h
is not included (and should not be included) which means there is no
MAX_PHYSMEM_BITS definition.

Missed that part :\, tks.

I don't think zsmalloc.c should be (ab)using MAX_PHYSMEM_BITS, and
your description above makes it sound like you expect it to always be
defined.

If we want to have a definition for this, we shouldn't be playing
fragile games like:

#ifndef MAX_POSSIBLE_PHYSMEM_BITS
#ifdef MAX_PHYSMEM_BITS
#define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
#else
/*
 * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
 * be PAGE_SHIFT
 */
#define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
#endif
#endif

but instead insist that MAX_PHYSMEM_BITS is defined _everywhere_.

Is it okay to propose using only MAX_PHYSMEM_BITS for zsmalloc (like
it was before commit 02390b87) instead, and make sure *at least* ARM
32/64 and x86/x64, for now, have it defined outside sparsemem headers
as well ? This way I can WARN_ONCE(), instead of BUG(), when specific
arch does not define it - enforcing behavior - showing BITS_PER_LONG
is being used instead of MAX_PHYSMEM_BITS (warning, at least once, for
the possibility of an overflow, like the issue showed in here).