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Subject: Re: [PATCH v2 1/4] mm/migration: reduce the rcu lock duration
To:     David Hildenbrand <david@redhat.com>
CC:     <ying.huang@intel.com>, <hch@lst.de>, <dhowells@redhat.com>,
        <cl@linux.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <akpm@linux-foundation.org>,
        <mike.kravetz@oracle.com>, <naoya.horiguchi@nec.com>
References: <20220425132723.34824-1-linmiaohe@huawei.com>
 <20220425132723.34824-2-linmiaohe@huawei.com>
 <eeda05fb-b0bb-3e1d-37e0-0021dd72e144@redhat.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <794b30f7-eea4-d254-9c1e-2fcd8a6aa16a@huawei.com>
Date:   Tue, 24 May 2022 20:36:52 +0800
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On 2022/4/29 17:54, David Hildenbrand wrote:
> On 25.04.22 15:27, Miaohe Lin wrote:
>> rcu_read_lock is required by grabbing the task refcount but it's not
>> needed for ptrace_may_access. So we could release the rcu lock after
>> task refcount is successfully grabbed to reduce the rcu holding time.
>>
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> Cc: Huang Ying <ying.huang@intel.com>
>> Cc: David Howells <dhowells@redhat.com>
>> Cc: Christoph Lameter <cl@linux.com>
>> ---
>>  mm/migrate.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index b2678279eb43..b779646665fe 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1902,17 +1902,16 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
>>  		return ERR_PTR(-ESRCH);
>>  	}
>>  	get_task_struct(task);
>> +	rcu_read_unlock();
>>  
>>  	/*
>>  	 * Check if this process has the right to modify the specified
>>  	 * process. Use the regular "ptrace_may_access()" checks.
>>  	 */
>>  	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
>> -		rcu_read_unlock();
>>  		mm = ERR_PTR(-EPERM);
>>  		goto out;
>>  	}
>> -	rcu_read_unlock();
>>  
>>  	mm = ERR_PTR(security_task_movememory(task));
>>  	if (IS_ERR(mm))
> 
> Similar pattern in:
> 
> mm/mempolicy.c:kernel_migrate_pages()
> kernel/futex/syscalls.c:get_robust_list()
> kernel/nsproxy.c:validate_nsset()
> 
> Exception:
> 
> sched/core_sched.c:sched_core_share_pid()
> 
> 
> Should we unify -- i.e., adjust the remaining 3 as well?
> 

I verified that this code change applies to kernel_migrate_pages(), but not get_robust_list()
and validate_nsset(). It's because task_struct reference is not grabbed for later ones. Will
send the new patch soon.

Thanks!