zsmalloc: remove insert_zspage() ->inuse optimization

Patch series "zsmalloc: fine-grained fullness and new compaction
algorithm", v4.

Existing zsmalloc page fullness grouping leads to suboptimal page
selection for both zs_malloc() and zs_compact().  This patchset reworks
zsmalloc fullness grouping/classification.

Additinally it also implements new compaction algorithm that is expected
to use less CPU-cycles (as it potentially does fewer memcpy-s in
zs_object_copy()).

Test (synthetic) results can be seen in patch 0003.


This patch (of 4):

This optimization has no effect.  It only ensures that when a zspage was
added to its corresponding fullness list, its "inuse" counter was higher
or lower than the "inuse" counter of the zspage at the head of the list. 
The intention was to keep busy zspages at the head, so they could be
filled up and moved to the ZS_FULL fullness group more quickly.  However,
this doesn't work as the "inuse" counter of a zspage can be modified by
obj_free() but the zspage may still belong to the same fullness list.  So,
fix_fullness_group() won't change the zspage's position in relation to the
head's "inuse" counter, leading to a largely random order of zspages
within the fullness list.

For instance, consider a printout of the "inuse" counters of the first 10
zspages in a class that holds 93 objects per zspage:

 ZS_ALMOST_EMPTY:  36  67  68  64  35  54  63  52

As we can see the zspage with the lowest "inuse" counter
is actually the head of the fullness list.

Remove this pointless "optimisation".

Link: https://lkml.kernel.org/r/20230304034835.2082479-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20230304034835.2082479-2-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zsmalloc.c

@@ -762,19 +762,8 @@ static void insert_zspage(struct size_class *class,
 				struct zspage *zspage,
 				enum fullness_group fullness)
 {
-	struct zspage *head;
-
 	class_stat_inc(class, fullness, 1);
-	head = list_first_entry_or_null(&class->fullness_list[fullness],
-					struct zspage, list);
-	/*
-	 * We want to see more ZS_FULL pages and less almost empty/full.
-	 * Put pages with higher ->inuse first.
-	 */
-	if (head && get_zspage_inuse(zspage) < get_zspage_inuse(head))
-		list_add(&zspage->list, &head->list);
-	else
-		list_add(&zspage->list, &class->fullness_list[fullness]);
+	list_add(&zspage->list, &class->fullness_list[fullness]);
 }
 
 /*