linux-kernel - Re: [PATCH 2/2] mm/gup: frame_vector: convert get_user_pages() --> pin_user

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Date:   Thu, 28 May 2020 10:46:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm/gup: frame_vector: convert get_user_pages() -->
 pin_user_pages()

On 28.05.20 00:32, John Hubbard wrote:
> This code was using get_user_pages*(), and all of the callers so far
> were in a "Case 2" scenario (DMA/RDMA), using the categorization
> from [1]. That means that it's time to convert the get_user_pages*() +
> put_page() calls to pin_user_pages*() + unpin_user_pages() calls.
> 
> There is some helpful background in [2]: basically, this is a small
> part of fixing a long-standing disconnect between pinning pages, and
> file systems' use of those pages.
> 
> [1] Documentation/core-api/pin_user_pages.rst
> 
> [2] "Explicit pinning of user-space pages":
>     https://lwn.net/Articles/807108/
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  mm/frame_vector.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index c431ca81dad5..4107dbca0056 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -72,7 +72,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>  	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
>  		vec->got_ref = true;
>  		vec->is_pfns = false;
> -		ret = get_user_pages_locked(start, nr_frames,
> +		ret = pin_user_pages_locked(start, nr_frames,
>  			gup_flags, (struct page **)(vec->ptrs), &locked);
>  		goto out;
>  	}
> @@ -122,7 +122,6 @@ EXPORT_SYMBOL(get_vaddr_frames);
>   */
>  void put_vaddr_frames(struct frame_vector *vec)
>  {
> -	int i;
>  	struct page **pages;
>  
>  	if (!vec->got_ref)
> @@ -135,8 +134,8 @@ void put_vaddr_frames(struct frame_vector *vec)
>  	 */
>  	if (WARN_ON(IS_ERR(pages)))
>  		goto out;
> -	for (i = 0; i < vec->nr_frames; i++)
> -		put_page(pages[i]);
> +
> +	unpin_user_pages(pages, vec->nr_frames);
>  	vec->got_ref = false;
>  out:
>  	vec->nr_frames = 0;
> 

Not familiar with this code, but from what I can tell this LGTM.

-- 
Thanks,

David / dhildenb