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From:   Muchun Song <songmuchun@bytedance.com>
To:     viro@zeniv.linux.org.uk, tj@kernel.org, axboe@fb.com,
        willy@infradead.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        duanxiongchun@bytedance.com,
        Muchun Song <songmuchun@bytedance.com>,
        Michal Hocko <mhocko@suse.com>
Subject: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css
Date:   Fri,  2 Apr 2021 17:11:45 +0800
Message-Id: <20210402091145.80635-1-songmuchun@bytedance.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

  rcu_read_lock()
  memcg_css = css_from_id()
  wb_get_create(memcg_css)
      cgwb_create(memcg_css)
          // css_get can change the ref counter from 0 back to 1
          css_get(memcg_css)
  rcu_read_unlock()

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.

Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Michal Hocko <mhocko@suse.com>
---
Changelog in v3:
 1. Do not change GFP_ATOMIC.
 2. Update commit log.

 Thanks for Michal's review and suggestions.

Changelog in v2:
 1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew.


 fs/fs-writeback.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3ac002561327..dedde99da40d 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -506,9 +506,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	/* 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_ATOMIC);
+	css_put(memcg_css);
 	if (!isw->new_wb)
 		goto out_free;
 
-- 
2.11.0