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Date:   Fri, 20 Aug 2021 10:16:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>,
        Xiyu Yang <xiyuyang19@fudan.edu.cn>,
        Alistair Popple <apopple@nvidia.com>,
        Yang Shi <shy828301@gmail.com>,
        Shakeel Butt <shakeelb@google.com>,
        Hugh Dickins <hughd@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, yuanxzhang@fudan.edu.cn,
        Xin Tan <tanxin.ctf@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        David Howells <dhowells@redhat.com>, keescook@chromium.org
Subject: Re: [PATCH] mm/rmap: Convert from atomic_t to refcount_t on
 anon_vma->refcount
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On Fri, Aug 20, 2021 at 08:43:40AM +0200, Peter Zijlstra wrote:

> Fine with me; although the immediate complaint from Andrew was about
> size, hence my UD1 hackery.
> 
> > So if we do this, I think it should be something like
> > 
> >    static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> >    {
> >         asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"
> >                 "jz %l[cc_zero]\n\t"
> >                 "jl %l[cc_error]"
> >                 : : [var] "m" (r->refs.counter)
> >                 : "memory" : cc_zero, cc_error);
> > 
> >         return false;
> > 
> >    cc_zero:
> >         return true;
> >    cc_error:
> >         refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
> >         return false;
> >    }
> > 
> > and we can discuss whether we could improve on the
> > refcount_warn_saturate() separately.
> 
> I can do the refcount_warn_saturate() change separately.
> 
> Let me go check how small I can get it...

gcc-10.2.1, x86_64-defconfig

kernel/event/core.o-inline-ud1:     96454
kernel/event/core.o-outofline-ud1:  96604
kernel/event/core.o-outofline-call: 97072

(42 refcount_warn_saturate/ud1 instances in that file,
 10 of which are refcount_dec_and_test)