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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Wed, 27 Oct 2021 15:46:42 +0800
Message-ID: <CAMZfGtVS2ZRAsd9HLODoLa1vVp0XCcBOTXjsmpoWwDcdGaADvQ@mail.gmail.com>
Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount
To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-m68k@lists.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        william.kucharski@oracle.com,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>, geert@linux-m68k.org,
        schmitzmic@gmail.com, Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, weixugc@google.com,
        Greg Thelen <gthelen@google.com>
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On Wed, Oct 27, 2021 at 1:38 AM Pasha Tatashin
<pasha.tatashin@soleen.com> 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.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>

I found some atomic_add/dec are replaced with atomic_add/dec_return,
those helpers with return value imply a full memory barrier around it, but
others without return value do not. Do you have any numbers to show
the impact? Maybe atomic_add/dec_return_relaxed can help this.

Thanks.