linux-kernel - Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->

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Date:   Wed, 26 Jan 2022 18:59:11 +0000
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, rientjes@...gle.com,
        pjt@...gle.com, hughd@...gle.com
Subject: Re: [PATCH v3 1/9] mm: add overflow and underflow checks for
 page->_refcount

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);
>  }