Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Oct 28, 2021 at 2:22 AM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> > 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

Right. There is no difference on x86. Maybe there are differences in
other architectures.

> used lock xadd instead of lock add. I am not sure if the performance
> difference is going to be different.
>
> Pasha