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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Mon, 14 Sep 2020 07:20:34 +0530
Message-ID: <CAFqt6zYru2VfaBFHEB6+7vMPXUbgX8R5f+GHqxaGjhP8QZEyzw@mail.gmail.com>
Subject: Re: [PATCH] mm/gup.c: Handling ERR within unpin_user_pages()
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org,
        John Hubbard <jhubbard@nvidia.com>
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On Sun, Sep 13, 2020 at 8:25 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Sun, Sep 13, 2020 at 08:02:35PM +0530, Souptick Joarder wrote:
> > It is possible that a buggy caller of unpin_user_pages()
> > (specially in error handling path) may end up calling it with
> > npages < 0 which is unnecessary.
> > @@ -328,6 +328,9 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
> >  {
> >       unsigned long index;
> >
> > +     if (WARN_ON_ONCE(npages < 0))
> > +             return;
>
> But npages is unsigned long.  So it can't be less than zero.

Sorry, I missed it.

Then, it means if npages is assigned with -ERRNO by caller, unpin_user_pages()
may end up calling a big loop, which is unnecessary.