linux-kernel - Re: Weird code with change "mm/gup: clean up follow_pfn

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Message-ID: <20220204012607.GQ1786498@nvidia.com>
Date:   Thu, 3 Feb 2022 21:26:07 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Peter Xu <peterx@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>, Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: Weird code with change "mm/gup: clean up follow_pfn_pte()
 slightly"

On Thu, Feb 03, 2022 at 05:22:36PM -0800, John Hubbard wrote:
> > So why do we return 0, if it means an error, instead of returning the
> > actual errno?
> 
> Well, now returning 0 sounds all wrong, when you put it like that. :)
> 
> So, simply this approach? :
> 
> @@ -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);

It is what I had in mind, I certainly wouldn't add a new condition
that returns 0..

Jason