Re: [PATCH 4/5] mm: gup: use get_user_pages_unlocked

[Peter Feiner]

On Wed, Oct 29, 2014 at 05:35:19PM +0100, Andrea Arcangeli wrote:
> This allows those get_user_pages calls to pass FAULT_FLAG_ALLOW_RETRY
> to the page fault in order to release the mmap_sem during the I/O.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reviewed-by: Peter Feiner <pfeiner@xxxxxxxxxx>

> diff --git a/net/ceph/pagevec.c b/net/ceph/pagevec.c
> index 5550130..096d914 100644
> --- a/net/ceph/pagevec.c
> +++ b/net/ceph/pagevec.c
> @@ -23,17 +23,15 @@ struct page **ceph_get_direct_page_vector(const void __user *data,
>  	if (!pages)
>  		return ERR_PTR(-ENOMEM);
>  
> -	down_read(&current->mm->mmap_sem);
>  	while (got < num_pages) {
> -		rc = get_user_pages(current, current->mm,
> +		rc = get_user_pages_unlocked(current, current->mm,
>  		    (unsigned long)data + ((unsigned long)got * PAGE_SIZE),
> -		    num_pages - got, write_page, 0, pages + got, NULL);
> +		    num_pages - got, write_page, 0, pages + got);
>  		if (rc < 0)
>  			break;
>  		BUG_ON(rc == 0);
>  		got += rc;
>  	}
> -	up_read(&current->mm->mmap_sem);
>  	if (rc < 0)
>  		goto fail;
>  	return pages;

I spent a while looking at this to make sure that BUG_ON(rc == 0) won't
trigger. AFAICT, __get_user_pages_locked can't return 0 since __get_user_pages
only returns 0 when nonblocking is not NULL; when __get_user_pages_locked
calls __get_user_pages with nonblocking != NULL (i.e., the first call in the
body of the for(;;) loop) and __get_user_pages returns 0, then
__get_user_pages_locked will call __get_user_pages again with nonblocking ==
NULL.
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