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In-Reply-To: <alpine.LSU.2.11.2007231549540.1016@eggly.anvils>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 23 Jul 2020 16:43:08 -0700
Message-ID: <CAHk-=wgvGOnMF0ePU4xS236bOsP8jouj3rps+ysCaGXvCjh2Dg@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>
Content-Type: multipart/mixed; boundary="00000000000027ca3205ab246ca4"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

--00000000000027ca3205ab246ca4
Content-Type: text/plain; charset="UTF-8"

On Thu, Jul 23, 2020 at 4:11 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Thu, 23 Jul 2020, Linus Torvalds wrote:
> >
> > I'll send a new version after I actually test it.
>
> I'll give it a try when you're happy with it.

Ok, what I described is what I've been running for a while now. But I
don't put much stress on my system with my normal workload, so..

> I did try yesterday's
> with my swapping loads on home machines (3 of 4 survived 16 hours),
> and with some google stresstests on work machines (0 of 10 survived).
>
> I've not spent long analyzing the crashes, all of them in or below
> __wake_up_common() called from __wake_up_locked_key_bookmark():
> sometimes gets to run the curr->func() and crashes on something
> inside there (often list_del's lib/list_debug.c:53!), sometimes
> cannot get that far. Looks like the wait queue entries on the list
> were not entirely safe with that patch.

Hmm. The bug Oleg pointed out should be pretty theoretical. But I
think the new approach with WQ_FLAG_WOKEN was much better anyway,
despite me missing that one spot in the first version of the patch.

So here's two patches - the first one does that wake_page_function()
conversion, and the second one just does the memory ordering cleanup I
mentioned.

I don't think the second one shouldn't matter on x86, but who knows.

I don't enable list debugging, but I find list corruption surprising.
All of _that_ should be inside the page waiqueue lock, the only
unlocked part was the "list_empty_careful()" part.

But I'll walk over my patch mentally one more time. Here's the current
version, anyway.

                Linus

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