From: Muchun Song songmuchun@bytedance.com
[ Upstream commit 8b0ed8443ae6458786580d36b7d5f8125535c5d4 ]
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") Link: https://lore.kernel.org/r/20210402091145.80635-1-songmuchun@bytedance.com Signed-off-by: Muchun Song songmuchun@bytedance.com Acked-by: Michal Hocko mhocko@suse.com Acked-by: Tejun Heo tj@kernel.org Signed-off-by: Jan Kara jack@suse.cz Signed-off-by: Sasha Levin sashal@kernel.org --- 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 fc9167e65287..869a34a48958 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -512,9 +512,14 @@ 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;