From: Muchun Song <songmuchun@bytedance.com>
To: Alexander Viro <viro@zeniv.linux.org.uk>,
Tejun Heo <tj@kernel.org>,
axboe@fb.com, Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Xiongchun duan <duanxiongchun@bytedance.com>,
Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css
Date: Thu, 20 May 2021 11:45:23 +0800 [thread overview]
Message-ID: <CAMZfGtUZgXsNOiyR==G+zLSN91PREss=XcbcfE0COkB8APcDxA@mail.gmail.com> (raw)
In-Reply-To: <20210402091145.80635-1-songmuchun@bytedance.com>
Hi,
It seems like this patch has not been added to the linux-next
tree. Can anyone help with this? Thanks.
On Fri, Apr 2, 2021 at 5:13 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> 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 prev parent reply other threads:[~2021-05-20 3:46 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-02 9:11 [PATCH v3] writeback: fix obtain a reference to a freeing memcg css Muchun Song
2021-04-04 18:01 ` Tejun Heo
2021-05-20 3:45 ` Muchun Song [this message]
2021-06-28 16:32 ` Jan Kara
2021-06-29 3:06 ` [External] " Muchun Song
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