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

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Date:   Thu, 3 Feb 2022 09:01:37 -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 12:38:33AM -0800, John Hubbard wrote:
> On 2/2/22 22:27, Lukas Bulwahn wrote:
> > Dear John,
> > 
> > Your change "mm/gup: clean up follow_pfn_pte() slightly" (see Link),
> > visible in linux-next as commit 05fef840b5c6 ("mm/gup: clean up
> > follow_pfn_pte() slightly"), is somehow weird.
> 
> Well. That sounds like something to be avoided. :)
> 
> > 
> > In the new branch if (pages), you set page = ERR_PTR(-EFAULT) and goto
> > out. However, at the label out, the value of page is not used, but the
> > return uses the variables i and ret.
> 
> Yes, I think that the complaint is accurate. The intent of this code is
> to return either number of pages so far (i) or ret (which should be zero
> in this case), because we are just stopping early, rather than calling
> this an actual error.

IIRC GUP shouldn't return 0, it should return an error code, not zero.

Jason