linux-kernel - Re: [PATCH 18/26] mm: Convert process_vm_rw_pages() to use get_user_pages

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Date:	Wed, 02 Oct 2013 12:32:33 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Jan Kara <jack@...e.cz>
CC:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	kosaki.motohiro@...il.com
Subject: Re: [PATCH 18/26] mm: Convert process_vm_rw_pages() to use get_user_pages_unlocked()

(10/2/13 10:27 AM), Jan Kara wrote:
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>   mm/process_vm_access.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
> index fd26d0433509..c1bc47d8ed90 100644
> --- a/mm/process_vm_access.c
> +++ b/mm/process_vm_access.c
> @@ -64,12 +64,8 @@ static int process_vm_rw_pages(struct task_struct *task,
>   	*bytes_copied = 0;
>   
>   	/* Get the pages we're interested in */
> -	down_read(&mm->mmap_sem);
> -	pages_pinned = get_user_pages(task, mm, pa,
> -				      nr_pages_to_copy,
> -				      vm_write, 0, process_pages, NULL);
> -	up_read(&mm->mmap_sem);
> -
> +	pages_pinned = get_user_pages_unlocked(task, mm, pa, nr_pages_to_copy,
> +					       vm_write, 0, process_pages);
>   	if (pages_pinned != nr_pages_to_copy) {
>   		rc = -EFAULT;
>   		goto end;

This is wrong because original code is wrong. In this function, page may be pointed to 
anon pages. Then, you should keep to take mmap_sem until finish to copying. Otherwise
concurrent fork() makes nasty COW issue.


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