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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Wed, 27 Oct 2021 14:22:08 -0400
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Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount
To:     Muchun Song <songmuchun@bytedance.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 Uytterhoeven <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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> I found some atomic_add/dec are replaced with atomic_add/dec_return,

I am going to replace -return variants with -fetch variants, potentially -fetch

> 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.

The generic variant uses  arch_cmpxchg() for all atomic variants
without any extra barriers. Therefore, on platforms that use generic
implementations there won't be performance differences except for an
extra branch that checks results when VM_BUG_ON is enabled.

On x86 the difference between the two is the following

atomic_add:
   lock add %eax,(%rsi)

atomic_fetch_add:
   lock xadd %eax,(%rsi)

atomic_fetch_add_relaxed:
   lock xadd %eax,(%rsi)

No differences between relaxed and non relaxed variants. However, we
used lock xadd instead of lock add. I am not sure if the performance
difference is going to be different.

Pasha