amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

[Hugh Dickins]

It's a bug in linux-next, but taking me too long to identify which
commit is "to blame", so let me throw it over to you without more
delay: I think __PageMovable() now needs to check !PageSlab().

I had made a small experimental change somewhere, rebuilt and rebooted,
was not surprised to crash once swapping and compaction came in,
but was surprised to find the crash in isolate_movable_page(),
called by compaction's isolate_migratepages_block().

page->mapping was ffffffff811303aa, which qualifies as __PageMovable(),
which expects struct movable_operations at page->mapping minus low bits.
But ffffffff811303aa was the address of SLUB's rcu_free_slab(): I have
CONFIG_CC_OPTIMIZE_FOR_SIZE=y, so function addresses may have low bits set.

Over to you! Thanks,
Hugh