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Date:   Sun, 13 Sep 2020 15:55:20 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Souptick Joarder <jrdr.linux@gmail.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH] mm/gup.c: Handling ERR within unpin_user_pages()
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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.