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Date:   Tue, 26 Oct 2021 20:48:59 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-m68k@lists.linux-m68k.org, anshuman.khandual@arm.com,
        akpm@linux-foundation.org, william.kucharski@oracle.com,
        mike.kravetz@oracle.com, vbabka@suse.cz, geert@linux-m68k.org,
        schmitzmic@gmail.com, rostedt@goodmis.org, mingo@redhat.com,
        hannes@cmpxchg.org, guro@fb.com, songmuchun@bytedance.com,
        weixugc@google.com, gthelen@google.com
Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for
 page->_refcount
Message-ID: <YXhbq/6OIpIAr7Tx@casper.infradead.org>
References: <20211026173822.502506-1-pasha.tatashin@soleen.com>
 <20211026173822.502506-2-pasha.tatashin@soleen.com>
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On Tue, Oct 26, 2021 at 05:38:15PM +0000, Pasha Tatashin wrote:
>  static inline void page_ref_add(struct page *page, int nr)
>  {
> -	atomic_add(nr, &page->_refcount);
> +	int ret;
> +
> +	VM_BUG_ON(nr <= 0);
> +	ret = atomic_add_return(nr, &page->_refcount);
> +	VM_BUG_ON_PAGE(ret <= 0, page);

This isn't right.  _refcount is allowed to overflow into the negatives.
See page_ref_zero_or_close_to_overflow() and the conversations that led
to it being added.