linux-kernel - Re: [RFC 1/8] mm: add overflow and underflow checks for page->

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Date:   Tue, 26 Oct 2021 20:48:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-m68k@...ts.linux-m68k.org, anshuman.khandual@....com,
        akpm@...ux-foundation.org, william.kucharski@...cle.com,
        mike.kravetz@...cle.com, vbabka@...e.cz, geert@...ux-m68k.org,
        schmitzmic@...il.com, rostedt@...dmis.org, mingo@...hat.com,
        hannes@...xchg.org, guro@...com, songmuchun@...edance.com,
        weixugc@...gle.com, gthelen@...gle.com
Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for
 page->_refcount

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.