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

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Date:   Tue, 12 Apr 2022 10:07:15 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.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 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/

Best Regards,
Huang, Ying