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Date: Tue, 2 Jul 2024 11:35:54 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar
 <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave Hansen
 <dave.hansen@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>, Andy
 Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Rick Edgecombe
 <rick.p.edgecombe@intel.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Subject: Re: [RESEND PATCH v3 0/3] x86/mm: LAM fixups and cleanups
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On Tue, 2 Jul 2024 10:39:03 -0700 Yosry Ahmed <yosryahmed@google.com> wrote:

> On Tue, Jul 2, 2024 at 10:36 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Tue,  2 Jul 2024 13:21:36 +0000 Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > > This series has fixups and cleanups for LAM. Most importantly, patch 1
> > > fixes a sycnhronization issue that may cause crashes of userspace
> > > applications. This is a resend of v3, rebased on top of v6.10-rc6.
> >
> > "Crashes of userspace applications" is bad.  Yet the patchset has been
> > floating about for four months.
> >
> > It's unclear (to me) how serious this is.  Can you please explain how
> > common this is, what the userspace application needs to do to trigger
> > this, etc?
> 
> I don't think it would be common. The bug only happens on new hardware
> supporting LAM, and it happens in a specific scenario where a
> userspace task enables LAM while a kthread is using (borrowing) its
> mm_struct on another CPU.
> 
> So it is possible but I certainly wouldn't call it common or easily triggerable.

But when people run older (or current) kernels on newer hardware, they
will hit this.  So a backport to cover 82721d8b25d7 ("x86/mm: Handle
LAM on context switch") is needed.

The series doesn't seem to be getting much traction so I can add it to
mm.git's mm-unstable branch for wider testing, but it's clearly an x86
tree thing.