lib/stackdepot: allow requesting early initialization dynamically

lib/stackdepot: allow requesting early initialization dynamically

In a later patch we want to add stackdepot support for object owner
tracking in slub caches, which is enabled by slub_debug boot parameter.
This creates a bootstrap problem as some caches are created early in
boot when slab_is_available() is false and thus stack_depot_init()
tries to use memblock. But, as reported by Hyeonggon Yoo [1] we are
already beyond memblock_free_all(). Ideally memblock allocation should
fail, yet it succeeds, but later the system crashes, which is a
separately handled issue.

To resolve this boostrap issue in a robust way, this patch adds another
way to request stack_depot_early_init(), which happens at a well-defined
point of time. In addition to build-time CONFIG_STACKDEPOT_ALWAYS_INIT,
code that's e.g. processing boot parameters (which happens early enough)
can call a new function stack_depot_want_early_init(), which sets a flag
that stack_depot_early_init() will check.

In this patch we also convert page_owner to this approach. While it
doesn't have the bootstrap issue as slub, it's also a functionality
enabled by a boot param and can thus request stack_depot_early_init()
with memblock allocation instead of later initialization with
kvmalloc().

As suggested by Mike, make stack_depot_early_init() only attempt
memblock allocation and stack_depot_init() only attempt kvmalloc().
Also change the latter to kvcalloc(). In both cases we can lose the
explicit array zeroing, which the allocations do already.

As suggested by Marco, provide empty implementations of the init
functions for !CONFIG_STACKDEPOT builds to simplify the callers.

[1] https://lore.kernel.org/all/YhnUcqyeMgCrWZbd@ip-172-31-19-208.ap-northeast-1.compute.internal/

Reported-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-and-tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>