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,
Muchun Song <songmuchun@bytedance.com>
Subject: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css
Date: Thu, 1 Apr 2021 17:33:43 +0800 [thread overview]
Message-ID: <20210401093343.51299-1-songmuchun@bytedance.com> (raw)
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@bytedance.com>
---
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
next reply other threads:[~2021-04-01 9:38 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 9:33 Muchun Song [this message]
2021-04-01 10:26 ` [PATCH v2] writeback: fix obtain a reference to a freeing memcg css Michal Hocko
2021-04-01 13:59 ` [External] " Muchun Song
2021-04-01 14:37 ` Michal Hocko
2021-04-01 14:39 ` Muchun Song
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