linux-kernel - Re: [PATCH v3 2/4] mm/gup: clean up follow_pfn

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Date:   Thu, 3 Feb 2022 14:53:52 +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>,
        Jan Kara <jack@...e.cz>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "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 v3 2/4] mm/gup: clean up follow_pfn_pte() slightly

On Thu 03-02-22 01:32:30, John Hubbard wrote:
> 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).
> 
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  mm/gup.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 65575ae3602f..cad3f28492e3 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -439,10 +439,6 @@ 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;
>  

This will also modify the error code returned from follow_page(). A quick
audit shows that at least the user in mm/migrate.c will propagate this
error code to userspace and I'm not sure the change in error code will not
break something... EEXIST is a bit strange error code to get from
move_pages(2).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR