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Date:   Tue, 4 Jul 2023 14:28:46 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Linux regressions mailing list <regressions@lists.linux.dev>,
        Bagas Sanjaya <bagasdotme@gmail.com>,
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        Linux Memory Management <linux-mm@kvack.org>,
        Linux PowerPC <linuxppc-dev@lists.ozlabs.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: Fwd: Memory corruption in multithreaded user space program
 while calling fork
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On Tue, 4 Jul 2023 13:22:54 -0700 Suren Baghdasaryan <surenb@google.com> wrote:

> On Tue, Jul 4, 2023 at 9:18 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Tue, 4 Jul 2023 09:00:19 +0100 Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > > > > > > Thanks! I'll investigate this later today. After discussing with
> > > > > > > Andrew, we would like to disable CONFIG_PER_VMA_LOCK by default until
> > > > > > > the issue is fixed. I'll post a patch shortly.
> > > > > >
> > > > > > Posted at: https://lore.kernel.org/all/20230703182150.2193578-1-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 older
> > > > versions. Did I miss something?
> > >
> > > 6.4.y is a stable kernel tree right now, so yes, it needs to be included
> > > there :)
> >
> > I'm in wait-a-few-days-mode on this.  To see if we have a backportable
> > 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 slightly
> regress, therefore disabling per-VMA locks by default for now seems to
> be preferable even with this fix (see discussion at
> https://lore.kernel.org/all/54cd9ffb-8f4b-003f-c2d6-3b6b0d2cb7d9@google.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@google.com/

This one isn't sufficient for .configs which already have
PER_VMA_LOCK=y.  Using `depends on BROKEN' would be better.

> [2] https://lore.kernel.org/all/20230704200656.2526715-1-surenb@google.com/
> 

We're still awaiting tester input on this?

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.

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.