Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sat, 25 Jul 2020 21:27:53 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Hugh Dickins <hughd@google.com>, Michal Hocko <mhocko@kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Michal Hocko <mhocko@suse.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page
Message-ID: <20200725192753.GA21962@redhat.com>
References: <20200723124749.GA7428@redhat.com>
 <CAHk-=wgyc7en4=HddEYiz_RKJXfqe1JYv3BzHc=+_wYq9ti+LQ@mail.gmail.com>
 <CAHk-=whQK3OGwExTzCrwwvuuVaQAgs8KsR-Yv8m1BmXoNZZ=jQ@mail.gmail.com>
 <alpine.LSU.2.11.2007231549540.1016@eggly.anvils>
 <CAHk-=wgvGOnMF0ePU4xS236bOsP8jouj3rps+ysCaGXvCjh2Dg@mail.gmail.com>
 <20200724152424.GC17209@redhat.com>
 <CAHk-=whuG+5pUeUqdiW4gk0prvqu7GZSMo-6oWv5PdDC5dBr=A@mail.gmail.com>
 <CAHk-=wjYHvbOs9i39EnUsC6VEJiuJ2e_5gZB5-J5CRKxq80B_Q@mail.gmail.com>
 <20200725101445.GB3870@redhat.com>
 <CAHk-=whSJbODMVmxxDs64f7BaESKWuMqOxWGpjUSDn6Jzqa71g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=whSJbODMVmxxDs64f7BaESKWuMqOxWGpjUSDn6Jzqa71g@mail.gmail.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Firstly, to avoid the confusion, let me repeat I think your patch is fine.

I too thought that non-exclusive waiters do not care about the bit state
and thus wake_page_function() can simply wake them all up.

But then I did "git blame", found your commit 3510ca20ece0150 and came to
conclusion there are reasons we should not do this.

On 07/25, Linus Torvalds wrote:
>
> On Sat, Jul 25, 2020 at 3:14 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > In essense, this partly reverts your commit 3510ca20ece0150
> > ("Minor page waitqueue cleanups"). I mean this part:
>
> Well, no. I mean, it keeps the "always add to the fail" behavior.

Ah, sorry for confusion, this doesn't matter. I didn't mean "fairness".

What I tried to say. AFAICS before that commit we had (almost) the same
behaviour you propose now: unlock_page/etc wakes all the non-exclusive
waiters up.

No?

Or I misunderstood your reply? Quite possibly, too late for me...

Oleg.