[PATCH 7/8] mm/page_io: don't nest queue_lock under rcu in bio_associate_blkg_from_page()

[Yu Kuai]

From: Yu Kuai <yukuai@xxxxxxx>

Take a css reference under RCU, drop RCU, and then associate the bio with
the blkg. This avoids nesting queue_lock under RCU and prepares to protect
blkcg with blkcg_mutex instead of queue_lock.

Use css_tryget() instead of css_tryget_online() so swap writeback for
pages charged to a dying memcg still passes the dying css to
bio_associate_blkg_from_css(). That preserves the existing closest-live
ancestor fallback instead of charging those bios to the root blkg.

Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
 mm/page_io.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 70cea9e24d2f..3b54c60c278e 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -315,12 +315,17 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct folio *folio)
 		return;
 
 	rcu_read_lock();
 	memcg = folio_memcg(folio);
 	css = cgroup_e_css(memcg->css.cgroup, &io_cgrp_subsys);
-	bio_associate_blkg_from_css(bio, css);
+	if (!css || !css_tryget(css))
+		css = NULL;
 	rcu_read_unlock();
+
+	bio_associate_blkg_from_css(bio, css);
+	if (css)
+		css_put(css);
 }
 #else
 #define bio_associate_blkg_from_page(bio, folio)		do { } while (0)
 #endif /* CONFIG_MEMCG && CONFIG_BLK_CGROUP */
 
-- 
2.51.0