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Date:   Tue, 26 Oct 2021 19:23:39 +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 0/8] Hardening page _refcount
Message-ID: <YXhHq52jDrU61V4E@casper.infradead.org>
References: <20211026173822.502506-1-pasha.tatashin@soleen.com>
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On Tue, Oct 26, 2021 at 05:38:14PM +0000, Pasha Tatashin wrote:
> It is hard to root cause _refcount problems, because they usually
> manifest after the damage has occurred.  Yet, they can lead to
> catastrophic failures such memory corruptions.
> 
> Improve debugability by adding more checks that ensure that
> page->_refcount never turns negative (i.e. double free does not
> happen, or free after freeze etc).
> 
> - Check for overflow and underflow right from the functions that
>   modify _refcount
> - Remove set_page_count(), so we do not unconditionally overwrite
>   _refcount with an unrestrained value
> - Trace return values in all functions that modify _refcount

I think this is overkill.  Won't we get exactly the same protection
by simply testing that page->_refcount == 0 in set_page_count()?
Anything which triggers that BUG_ON would already be buggy because
it can race with speculative gets.