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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 4 Jul 2023 23:42:19 -0700
Message-ID: <CAJuCfpFkB3OoahH6URzWi+4L50daNGqDVPeUaF5GGuez5tLk8w@mail.gmail.com>
Subject: Re: Fwd: Memory corruption in multithreaded user space program while
 calling fork
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Linux regressions mailing list <regressions@lists.linux.dev>,
        Bagas Sanjaya <bagasdotme@gmail.com>,
        Jacob Young <jacobly.alt@gmail.com>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Memory Management <linux-mm@kvack.org>,
        Linux PowerPC <linuxppc-dev@lists.ozlabs.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Tue, Jul 4, 2023 at 3:04=E2=80=AFPM Suren Baghdasaryan <surenb@google.co=
m> wrote:
>
> On Tue, Jul 4, 2023 at 2:28=E2=80=AFPM Andrew Morton <akpm@linux-foundati=
on.org> wrote:
> >
> > On Tue, 4 Jul 2023 13:22:54 -0700 Suren Baghdasaryan <surenb@google.com=
> wrote:
> >
> > > On Tue, Jul 4, 2023 at 9:18=E2=80=AFAM Andrew Morton <akpm@linux-foun=
dation.org> wrote:
> > > >
> > > > On Tue, 4 Jul 2023 09:00:19 +0100 Greg KH <gregkh@linuxfoundation.o=
rg> wrote:
> > > >
> > > > > > > > > Thanks! I'll investigate this later today. After discussi=
ng with
> > > > > > > > > Andrew, we would like to disable CONFIG_PER_VMA_LOCK by d=
efault until
> > > > > > > > > the issue is fixed. I'll post a patch shortly.
> > > > > > > >
> > > > > > > > Posted at: https://lore.kernel.org/all/20230703182150.21935=
78-1-surenb@google.com/
> > > > > > >
> > > > > > > As that change fixes something in 6.4, why not cc: stable on =
it as well?
> > > > > >
> > > > > > Sorry, I thought since per-VMA locks were introduced in 6.4 and=
 this
> > > > > > patch is fixing 6.4 I didn't need to send it to stable for olde=
r
> > > > > > versions. Did I miss something?
> > > > >
> > > > > 6.4.y is a stable kernel tree right now, so yes, it needs to be i=
ncluded
> > > > > there :)
> > > >
> > > > I'm in wait-a-few-days-mode on this.  To see if we have a backporta=
ble
> > > > fix rather than disabling the feature in -stable.
> > >
> > > Ok, I think we have a fix posted at [2]  and it's cleanly applies to
> > > 6.4.y stable branch as well. However fork() performance might slightl=
y
> > > regress, therefore disabling per-VMA locks by default for now seems t=
o
> > > be preferable even with this fix (see discussion at
> > > https://lore.kernel.org/all/54cd9ffb-8f4b-003f-c2d6-3b6b0d2cb7d9@goog=
le.com/).
> > > IOW, both [1] and [2] should be applied to 6.4.y stable. Both apply
> > > cleanly and I CC'ed stable on [2]. Greg, should I send [1] separately
> > > to stable@vger?
> > >
> > > [1] https://lore.kernel.org/all/20230703182150.2193578-1-surenb@googl=
e.com/
> >
> > This one isn't sufficient for .configs which already have
> > PER_VMA_LOCK=3Dy.  Using `depends on BROKEN' would be better.
> >
> > > [2] https://lore.kernel.org/all/20230704200656.2526715-1-surenb@googl=
e.com/
> > >
> >
> > We're still awaiting tester input on this?
>
> Yeah, and it seems to be negative... Anyway, I'll post a dependency on BR=
OKEN.

I posted the patchset at
https://lore.kernel.org/all/20230705063711.2670599-1-surenb@google.com/
CC'ing stable@vger with the cover letter explaining the situation. The
negative report might have been a fluke, so let's wait for more
testing. In the meantime we can disable the feature by applying the
last patch in that series.

>
> >
> > I think a clean new fully-changelogged two-patch series would be the
> > best way to handle this.  Please ensure that the [0/2] intro clearly
> > explains what we're proposing here, and why.

Done.

> >
> > Also, "might slightly regress" is a bit weak.  These things are
> > measurable, no?  Because a better solution would be to fix 6.4.x and
> > mainline and leave it at that.

They are measurable and they were included in the fix I posted. I
added the numbers in the new cover letter as well.
Thanks,
Suren.

> >