mm: mglru: fix stale batch updates after memcg reparenting

mm: mglru: fix stale batch updates after memcg reparenting

The mglru page table walker batches per-generation size deltas in
walk->nr_pages while walking page tables without holding the lruvec lock.
The reset_batch_size() later folds those deltas into walk->lruvec under
the lruvec lock.

The page table walker can run concurrently with the memcg reparenting path
as follows:

CPU0                           CPU1
====                           ====

walk_mm
--> walk_page_range
    --> update_batch_size
        --> walk->nr_pages += delta

                              mem_cgroup_css_offline
                              --> memcg_reparent_objcgs
                                  --> lock lruvec
                                      lru_gen_reparent_memcg
                                      --> reparent child folios to parent
                                      unlock lruvec

    lock lruvec
    reset_batch_size
    --> child lrugen->nr_pages += delta

This will trigger the following warning in lru_gen_exit_memcg():

	VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
				   sizeof(lruvec->lrugen.nr_pages)));

And the user-visible impact of underestimated nr_pages in MGLRU was
premature OOMs because MGLRU does not try to reclaim memory when nr_pages
reaches zero, but there are still more pages.

To fix it, make reset_batch_size() check CSS_DYING under RCU before
flushing the pending batch.  A non-dying memcg keeps the original lruvec
stable against RCU-delayed offlining; a dying memcg redirects the deltas
to the first non-dying ancestor.

Link: https://lore.kernel.org/20260710154318.75388-1-qi.zheng@linux.dev
Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reported-by: Peiyang He <peiyang_he@smail.nju.edu.cn>
Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>