linux-kernel - Re: [PATCH 03/16] mm: simplify gup_fast

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Date:   Mon, 3 Jun 2019 09:41:21 +0200
From:   Christoph Hellwig <hch@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Nicholas Piggin <npiggin@...il.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-mips@...r.kernel.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Linux-MM <linux-mm@...ck.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/16] mm: simplify gup_fast_permitted

On Sat, Jun 01, 2019 at 09:14:17AM -0700, Linus Torvalds wrote:
> On Sat, Jun 1, 2019 at 12:50 AM Christoph Hellwig <hch@....de> wrote:
> >
> > Pass in the already calculated end value instead of recomputing it, and
> > leave the end > start check in the callers instead of duplicating them
> > in the arch code.
> 
> Good cleanup, except it's wrong.
> 
> > -       if (nr_pages <= 0)
> > +       if (end < start)
> >                 return 0;
> 
> You moved the overflow test to generic code - good.
> 
> You removed the sign and zero test on nr_pages - bad.

I only removed a duplicate of it.  The full (old) code in
get_user_pages_fast() looks like this:

	if (nr_pages <= 0)
		return 0;

	if (unlikely(!access_ok((void __user *)start, len)))
		return -EFAULT;

	if (gup_fast_permitted(start, nr_pages)) {