[PATCH v2] writeback: fix obtain a reference to a freeing memcg css

[Muchun Song]

The caller of wb_get_create() should pin the memcg, because
wb_get_create() relies on this guarantee. The rcu read lock
only can guarantee that the memcg css returned by css_from_id()
cannot be released, but the reference of the memcg can be zero.
Fix it by holding a reference to the css before calling
wb_get_create(). This is not a problem I encountered in the
real world. Just the result of a code review.

And it is unnecessary to use GFP_ATOMIC, so replace it with
GFP_NOIO.

Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
Changelog in v2:
 1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew.

 fs/fs-writeback.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e91980f49388..df7f89f8f771 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -501,16 +501,21 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	if (atomic_read(&isw_nr_in_flight) > WB_FRN_MAX_IN_FLIGHT)
 		return;
 
-	isw = kzalloc(sizeof(*isw), GFP_ATOMIC);
+	isw = kzalloc(sizeof(*isw), GFP_NOIO);
 	if (!isw)
 		return;
 
 	/* find and pin the new wb */
 	rcu_read_lock();
 	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
-	if (memcg_css)
-		isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
+	if (memcg_css && !css_tryget(memcg_css))
+		memcg_css = NULL;
 	rcu_read_unlock();
+	if (!memcg_css)
+		goto out_free;
+
+	isw->new_wb = wb_get_create(bdi, memcg_css, GFP_NOIO);
+	css_put(memcg_css);
 	if (!isw->new_wb)
 		goto out_free;
 
-- 
2.11.0