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 [thread overview]
Message-ID: <20210402091145.80635-1-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.
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
next reply other threads:[~2021-04-02 9:13 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-02 9:11 Muchun Song [this message]
2021-04-04 18:01 ` [PATCH v3] writeback: fix obtain a reference to a freeing memcg css Tejun Heo
2021-05-20 3:45 ` Muchun Song
2021-06-28 16:32 ` Jan Kara
2021-06-29 3:06 ` [External] " Muchun Song
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210402091145.80635-1-songmuchun@bytedance.com \
--to=songmuchun@bytedance.com \
--cc=axboe@fb.com \
--cc=duanxiongchun@bytedance.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mhocko@suse.com \
--cc=tj@kernel.org \
--cc=viro@zeniv.linux.org.uk \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).