mm: remove '!root_reclaim' checking in should_abort_scan()

Android systems usually use memory.reclaim interface to implement user
space memory management which expects that the requested reclaim target
and actually reclaimed amount memory are not diverging by too much. With
the current MGRLU implementation there is, however, no bail out when the
reclaim target is reached and this could lead to an excessive reclaim
that scales with the reclaim hierarchy size.For example, we can get a
nr_reclaimed=394/nr_to_reclaim=32 proactive reclaim under a common 1-N
cgroup hierarchy.

This defect arose from the goal of keeping fairness among memcgs that is,
for try_to_free_mem_cgroup_pages -> shrink_node_memcgs -> shrink_lruvec ->
lru_gen_shrink_lruvec -> try_to_shrink_lruvec, the !root_reclaim(sc) check
was there for reclaim fairness, which was necessary before commit
b82b530 ("mm: vmscan: restore incremental cgroup iteration") because
the fairness depended on attempted proportional reclaim from every memcg
under the target memcg.  However after commit b82b530 there is no
longer a need to visit every memcg to ensure fairness.  Let's have
try_to_shrink_lruvec bail out when the nr_reclaimed achieved.

Link: https://lore.kernel.org/20260318011558.1696310-1-zhaoyang.huang@unisoc.com
Link: https://lore.kernel.org/20260212032111.408865-1-zhaoyang.huang@unisoc.com
Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Suggested-by: T.J.Mercier <tjmercier@google.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Zhaoyang Huang

mm/vmscan.c

@@ -4971,10 +4971,6 @@ static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
 	int i;
 	enum zone_watermarks mark;
 
-	/* don't abort memcg reclaim to ensure fairness */
-	if (!root_reclaim(sc))
-		return false;
-
 	if (sc->nr_reclaimed >= max(sc->nr_to_reclaim, compact_gap(sc->order)))
 		return true;