linux-kernel - Re: [PATCH 6/9] mm/z3fold: move decrement of pool->pages_nr into __release_z3fold

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Message-ID: <CAM4kBB+oHqn=AAqKrxgN=e7iyRiZs0HDx8J585Vugf4kyWfF5Q@mail.gmail.com>
Date:   Wed, 2 Mar 2022 11:19:21 +0100
From:   Vitaly Wool <vitaly.wool@...sulko.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/9] mm/z3fold: move decrement of pool->pages_nr into __release_z3fold_page()

On Wed, Mar 2, 2022 at 10:12 AM David Laight <David.Laight@...lab.com> wrote:
>
> > > Atomic operations aren't magic.
> > > Atomic operations are (at best) one slow locked bus cycle.
> > > Acquiring a lock is the same.
> > > Releasing a lock might be cheaper, but is probably a locked bus cycle.
> > >
> > > So if you use state_lock to protect pages_nr then you lose an atomic
> > > operation for the decrement and gain one (for the unlock) in the increment.
> > > That is even or maybe a slight gain.
> > > OTOH a 64bit atomic is a PITA on some 32bit systems.
> > > (In fact any atomic is a PITA on sparc32.)
> >
> > It's actually *stale_lock* and it's very misleading to use it for this.
> > I would actually like to keep atomics but I have no problem with
> > making it 32-bit for 32-bit systems. Would that work for you guys?
>
> It would be better to rename the lock.

No it would not because that lock is protecting the list of entries
that could not be immediately freed.

~Vitaly