Re: [PATCH v3 1/4] mm: reduce the rcu lock duration

[Oscar Salvador <osalvador@suse.de>]

On Wed, May 25, 2022 at 04:18:19PM +0800, Miaohe Lin wrote:
> Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct")
> extends the period of the rcu_read_lock until after the permissions checks
> are done to prevent the task pointed to from changing from under us. But
> the task_struct refcount is also taken at that time, the reference to task
> is guaranteed to be stable. So it's unnecessary to extend the period of
> the rcu_read_lock. 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>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

> ---
>  mm/mempolicy.c | 3 +--
>  mm/migrate.c   | 3 +--
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 0b4ba3ee810e..2dad094177bf 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1609,6 +1609,7 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
>  		goto out;
>  	}
>  	get_task_struct(task);
> +	rcu_read_unlock();
>  
>  	err = -EINVAL;
>  
> @@ -1617,11 +1618,9 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
>  	 * Use the regular "ptrace_may_access()" checks.
>  	 */
>  	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
> -		rcu_read_unlock();
>  		err = -EPERM;
>  		goto out_put;
>  	}
> -	rcu_read_unlock();
>  
>  	task_nodes = cpuset_mems_allowed(task);
>  	/* Is the user allowed to access the target nodes? */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e51588e95f57..e88ebb88fa6f 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))
> -- 
> 2.23.0
> 
> 

-- 
Oscar Salvador
SUSE Labs