linux-kernel - Re: [PATCH v2 1/4] mm/migration: reduce the rcu lock duration

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Message-ID: <eeda05fb-b0bb-3e1d-37e0-0021dd72e144@redhat.com>
Date:   Fri, 29 Apr 2022 11:54:36 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, naoya.horiguchi@....com
Cc:     ying.huang@...el.com, hch@....de, dhowells@...hat.com,
        cl@...ux.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm/migration: reduce the rcu lock duration

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@...edance.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Christoph Lameter <cl@...ux.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?

-- 
Thanks,

David / dhildenb