In the next commit we want to rearrange struct slab fields to allow a larger
rcu_head. Afterwards, the page->mapping field will overlap with SLUB's "struct
list_head slab_list", where the value of prev pointer can become LIST_POISON2,
which is 0x122 + POISON_POINTER_DELTA.  Unfortunately the bit 1 being set can
confuse PageMovable() to be a false positive and cause a GPF as reported by lkp
[1].

To fix this, make isolate_movable_page() skip pages with the PageSlab flag set.
This is a bit tricky as we need to add memory barriers to SLAB and SLUB's page
allocation and freeing, and their counterparts to isolate_movable_page().

Based on my RFC from [2]. Added a comment update from Matthew's variant in [3]
and, as done there, moved the PageSlab checks to happen before trying to take
the page lock.

[1] https://lore.kernel.org/all/208c1757-5edd-fd42-67d4-1940cc43b50f@intel.com/
[2] https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@suse.cz/
[3] https://lore.kernel.org/all/YzsVM8eToHUeTP75@casper.infradead.org/Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>