linux-kernel - Re: [PATCH 1/4] mm/migration: reduce the rcu lock duration

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Message-ID: <34f3aff7-0111-a8d6-a559-92bf6d0bd62a@huawei.com>
Date:   Tue, 12 Apr 2022 11:21:31 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     "ying.huang@...el.com" <ying.huang@...el.com>,
        <akpm@...ux-foundation.org>
CC:     <mike.kravetz@...cle.com>, <shy828301@...il.com>,
        <willy@...radead.org>, <ziy@...dia.com>, <minchan@...nel.org>,
        <apopple@...dia.com>, <dave.hansen@...ux.intel.com>,
        <o451686892@...il.com>, <jhubbard@...dia.com>, <peterx@...hat.com>,
        <naoya.horiguchi@....com>, <mhocko@...e.com>, <riel@...hat.com>,
        <osalvador@...e.de>, <david@...hat.com>, <sfr@...b.auug.org.au>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] mm/migration: reduce the rcu lock duration

On 2022/4/12 10:07, ying.huang@...el.com wrote:
> On Sat, 2022-04-09 at 15:38 +0800, 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@...edance.com>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/migrate.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index a3d8c2be2631..d8aae6c75990 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1907,17 +1907,16 @@ 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))
> 
> Why do you ignore our discussion for the previous version?
> 
> https://lore.kernel.org/linux-mm/8735ju7as9.fsf@yhuang6-desk2.ccr.corp.intel.com/
> 

Sorry for confusing. I remember this patch is pending for verify. The reason I post this series is that
I want to move the other patches in this series forward while this patch is still pending for verify.

Thanks.

> Best Regards,
> Huang, Ying
> 
> 
> .
>