linux-kernel - Re: [PATCH v2 1/3] mm/gup_benchmark: handle gup failures

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Message-Id: <20180409173821.889f0a2dd4385ee2428c16b8@linux-foundation.org>
Date:   Mon, 9 Apr 2018 17:38:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        stable <stable@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/3] mm/gup_benchmark: handle gup failures

On Sun, 8 Apr 2018 06:12:13 +0300 "Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Sat, Apr 07, 2018 at 01:08:43PM -0700, Linus Torvalds wrote:
> > On Thu, Apr 5, 2018 at 2:03 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > >                 nr = get_user_pages_fast(addr, nr, gup->flags & 1, pages + i);
> > > -               i += nr;
> > > +               if (nr > 0)
> > > +                       i += nr;
> > 
> > Can we just make this robust while at it, and just make it
> > 
> >         if (nr <= 0)
> >                 break;
> > 
> > instead? Then it doesn't care about zero vs negative error, and
> > wouldn't get stuck in an endless loop if it got zero.
> > 
> >              Linus
> 
> I don't mind though it alredy breaks out on the next cycle:
> 
>                 if (nr != gup->nr_pages_per_call)
>                         break;
> 
> the only issue is i getting corrupted when nr < 0;
> 

It does help readability to have the thing bail out as soon as we see
something go bad.  This?

--- a/mm/gup_benchmark.c~mm-gup_benchmark-handle-gup-failures-fix
+++ a/mm/gup_benchmark.c
@@ -41,8 +41,9 @@ static int __gup_benchmark_ioctl(unsigne
 		}
 
 		nr = get_user_pages_fast(addr, nr, gup->flags & 1, pages + i);
-		if (nr > 0)
-			i += nr;
+		if (nr <= 0)
+			break;
+		i += nr;
 	}
 	end_time = ktime_get();
 
_