linux-kernel - Re: [RESEND PATCH v3 0/3] x86/mm: LAM fixups and cleanups

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Date: Tue, 2 Jul 2024 10:39:03 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v3 0/3] x86/mm: LAM fixups and cleanups

On Tue, Jul 2, 2024 at 10:36 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Tue,  2 Jul 2024 13:21:36 +0000 Yosry Ahmed <yosryahmed@...gle.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.