Currently we reach inside the list_head.  This is a violation of the layer
of abstraction provided by the list_head.  It makes the code fragile.
More importantly it makes the code wicked hard to understand.

The code reaches into the list_head structure to counteract the fact that
the list _may_ have been changed during slob_page_alloc().  Instead of
this we can add a return parameter to slob_page_alloc() to signal that the
list was modified (list_del() called with page->lru to remove page from
the freelist).

This code is concerned with an optimisation that counters the tendency for
first fit allocation algorithm to fragment memory into many small chunks
at the front of the memory pool.  Since the page is only removed from the
list when an allocation uses _all_ the remaining memory in the page then
in this special case fragmentation does not occur and we therefore do not
need the optimisation.

Add a return parameter to slob_page_alloc() to signal that the allocation
used up the whole page and that the page was removed from the free list.
After calling slob_page_alloc() check the return value just added and only
attempt optimisation if the page is still on the list.

Use list_head API instead of reaching into the list_head structure to
check if sp is at the front of the list.

Link: http://lkml.kernel.org/r/20190402230545.2929-3-tobin@kernel.orgSigned-off-by: Tobin C. Harding <tobin@kernel.org>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>