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In-Reply-To: <alpine.LSU.2.11.2008052105040.8716@eggly.anvils>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 6 Aug 2020 10:07:07 -0700
Message-ID: <CAHk-=whf7wCUV2oTDUg0eeNafhhk_OhJBT2SbHZXwgtmAzNeTg@mail.gmail.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page
To:     Hugh Dickins <hughd@google.com>
Cc:     Oleg Nesterov <oleg@redhat.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>,
        Greg KH <gregkh@linuxfoundation.org>
Content-Type: multipart/mixed; boundary="000000000000a9dffa05ac388522"
Sender: linux-kernel-owner@vger.kernel.org
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--000000000000a9dffa05ac388522
Content-Type: text/plain; charset="UTF-8"

On Wed, Aug 5, 2020 at 10:21 PM Hugh Dickins <hughd@google.com> wrote:
>
> Something I was interested to realize in looking at this: trylock_page()
> on a contended lock is now much less likely to jump the queue and
> succeed than before, since your lock holder hands off the page lock to
> the next holder: much smaller window than waiting for the next to wake
> to take it. Nothing wrong with that, but effect might be seen somewhere.

Yeah, the window is smaller, but it's not gone.

It *might* be interesting to actually do the handover directly from
"unlock_page()", and avoid clearing (and then setting) the bit
entirely.

Something like the attached TOTALLY UNTESTED patch.

NOTE! Sometimes when I say something is untested, I think the patch is
fine because it's simple and straightforward, I just didn't test it.

This time it's both untested and very very subtle indeed. Did I get
the hand-over case SMP memory barriers right? Did I screw something
else up?

So this might be complete garbage. I really don't know. But it might
close the window for an unfair trylock (or lucky page_lock())
entirely.

Or maybe it just makes page locking break entirely. It's a very real risk.

The upside may not be worth it.

               Linus

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