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Date:   Wed, 26 Jan 2022 18:59:11 +0000
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, rientjes@google.com,
        pjt@google.com, hughd@google.com
Subject: Re: [PATCH v3 1/9] mm: add overflow and underflow checks for
 page->_refcount
Message-ID: <YfGZ/0hhkqVeX04p@casper.infradead.org>
References: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
 <20220126183429.1840447-2-pasha.tatashin@soleen.com>
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On Wed, Jan 26, 2022 at 06:34:21PM +0000, Pasha Tatashin wrote:
> The problems with page->_refcount are hard to debug, because usually
> when they are detected, the damage has occurred a long time ago. Yet,
> the problems with invalid page refcount may be catastrophic and lead to
> memory corruptions.
> 
> Reduce the scope of when the _refcount problems manifest themselves by
> adding checks for underflows and overflows into functions that modify
> _refcount.

If you're chasing a bug like this, presumably you turn on page
tracepoints.  So could we reduce the cost of this by putting the
VM_BUG_ON_PAGE parts into __page_ref_mod() et al?  Yes, we'd need to
change the arguments to those functions to pass in old & new, but that
should be a cheap change compared to embedding the VM_BUG_ON_PAGE.

>  static inline void page_ref_add(struct page *page, int nr)
>  {
> -	atomic_add(nr, &page->_refcount);
> +	int old_val = atomic_fetch_add(nr, &page->_refcount);
> +	int new_val = old_val + nr;
> +
> +	VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
>  	if (page_ref_tracepoint_active(page_ref_mod))
>  		__page_ref_mod(page, nr);
>  }