linux-kernel - Re: [PATCH v4 2/5] mm/gup: follow_pfn

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Date:   Fri, 4 Feb 2022 12:41:37 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        David Hildenbrand <david@...hat.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v4 2/5] mm/gup: follow_pfn_pte(): -EEXIST cleanup

On Thu 03-02-22 18:00:07, John Hubbard wrote:
> Remove a quirky special case from follow_pfn_pte(), and adjust its
> callers to match. Caller changes include:
> 
> __get_user_pages(): Regardless of any FOLL_* flags, get_user_pages() and
> its variants should handle PFN-only entries by stopping early, if the
> caller expected **pages to be filled in. This makes for a more reliable
> API, as compared to the previous approach of skipping over such entries
> (and thus leaving them silently unwritten).
> 
> move_pages(): squash the -EEXIST error return from follow_page() into
> -EFAULT, because -EFAULT is listed in the man page, whereas -EEXIST is
> not.
> 
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  mm/gup.c     | 13 ++++++++-----
>  mm/migrate.c |  7 +++++++
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 80229ecf0114..2df0d0103c43 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -464,10 +464,6 @@ static struct page *no_page_table(struct vm_area_struct *vma,
>  static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
>  		pte_t *pte, unsigned int flags)
>  {
> -	/* No page to get reference */
> -	if (flags & (FOLL_GET | FOLL_PIN))
> -		return -EFAULT;
> -
>  	if (flags & FOLL_TOUCH) {
>  		pte_t entry = *pte;
>  
> @@ -1205,8 +1201,15 @@ static long __get_user_pages(struct mm_struct *mm,
>  		} else if (PTR_ERR(page) == -EEXIST) {
>  			/*
>  			 * Proper page table entry exists, but no corresponding
> -			 * struct page.
> +			 * struct page. If the caller expects **pages to be
> +			 * filled in, bail out now, because that can't be done
> +			 * for this page.
>  			 */
> +			if (pages) {
> +				ret = PTR_ERR(page);
> +				goto out;
> +			}
> +
>  			goto next_page;
>  		} else if (IS_ERR(page)) {
>  			ret = PTR_ERR(page);
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c7da064b4781..be0d5ae36dc1 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1761,6 +1761,13 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  			continue;
>  		}
>  
> +		/*
> +		 * The move_pages() man page does not have an -EEXIST choice, so
> +		 * use -EFAULT instead.
> +		 */
> +		if (err == -EEXIST)
> +			err = -EFAULT;
> +
>  		/*
>  		 * If the page is already on the target node (!err), store the
>  		 * node, otherwise, store the err.
> -- 
> 2.35.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR