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Date:   Thu, 7 Dec 2023 08:21:21 -0500
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Subject: Re: [PATCH-cgroup] cgroup: Move rcu_head up near the top of
 cgroup_root
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To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Yafang Shao <laoar.shao@gmail.com>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Stephen Rothwell <sfr@canb.auug.org.au>
References: <20231207043753.876437-1-longman@redhat.com>
 <CAJD7tkZtt8xedBJyRns+6HpdXoBxadLUGuGNG5s1trEbRgb9hA@mail.gmail.com>
From:   Waiman Long <longman@redhat.com>
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On 12/6/23 23:51, Yosry Ahmed wrote:
> On Wed, Dec 6, 2023 at 8:38 PM Waiman Long <longman@redhat.com> wrote:
>> Commit d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU
>> safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
>> for freeing the cgroup_root.
>>
>> The use of kvfree_rcu(), however, has the limitation that the offset of
>> the rcu_head structure within the larger data structure cannot exceed
>> 4096 or the compilation will fail. By putting rcu_head below the cgroup
>> structure, any change to the cgroup structure that makes it larger has
>> the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
>> cacheline sharing of read mostly rstat_cpu") happens to be the commit
>> that breaks it even though it is not its fault. Fix it by moving the
>> rcu_head structure up before the cgroup structure.
>>
>> Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   include/linux/cgroup-defs.h | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
>> index 5a97ea95b564..45359969d8cf 100644
>> --- a/include/linux/cgroup-defs.h
>> +++ b/include/linux/cgroup-defs.h
>> @@ -562,6 +562,10 @@ struct cgroup_root {
>>          /* Unique id for this hierarchy. */
>>          int hierarchy_id;
>>
>> +       /* A list running through the active hierarchies */
>> +       struct list_head root_list;
>> +       struct rcu_head rcu;
>> +
> Perhaps the comment should mention the placement requirements, and
> maybe a pointer to wherever it is specified that the offset of struct
> rcu_head should not exceed 4096?

Fair. I will update the patch description to give more details about 
that. It is new to me too before I got a compilation failure report in 
linux-next.

Cheers,
Longman

>